The mole
Mole
The unit of amount of substance, symbol mol\text{mol}mol, where one mole of a substance contains 6.02×10236.02 \times 10^{23}6.02×1023 particles.
Avogadro constant
The number of particles, such as atoms, ions or molecules, in one mole of a substance, equal to 6.02×10236.02 \times 10^{23}6.02×1023.
- Atoms are far too small to count one by one, so chemists count them in very large, fixed-size groups called moles.
- The mole, symbol mol\text{mol}mol, is the unit of amount of substance.
- One mole of any substance contains 6.02×10236.02 \times 10^{23}6.02×1023 particles, and this number is the Avogadro constant.
- The particles being counted can be atoms, ions or molecules, so you must always say which particle you mean.
- One mole of helium contains 6.02×10236.02 \times 10^{23}6.02×1023 atoms, while one mole of carbon dioxide contains 6.02×10236.02 \times 10^{23}6.02×1023 molecules.
- The number of particles in a sample is found using number of particles=amount (mol)×6.02×1023\text{number of particles} = \text{amount (mol)} \times 6.02 \times 10^{23}number of particles=amount (mol)×6.02×1023.
- Rearranging gives amount (mol)=number of particles6.02×1023\text{amount (mol)} = \dfrac{\text{number of particles}}{6.02 \times 10^{23}}amount (mol)=6.02×1023number of particles.
- One mole of a compound can contain more than one mole of a particular atom or ion, depending on its formula.
- One mole of H2O\text{H}_2\text{O}H2O contains 2 mol2\ \text{mol}2 mol of hydrogen atoms and 1 mol1\ \text{mol}1 mol of oxygen atoms.
- One mole of CaCl2\text{CaCl}_2CaCl2 contains 1 mol1\ \text{mol}1 mol of Ca2+\text{Ca}^{2+}Ca2+ ions and 2 mol2\ \text{mol}2 mol of Cl−\text{Cl}^-Cl− ions.
Counting particles
- A sample contains 0.250 mol0.250\ \text{mol}0.250 mol of water molecules.
- Each water molecule contains three atoms, so the number of atoms is three times larger.
Molar mass
Molar mass
The mass of one mole of a substance, measured in g/mol\text{g/mol}g/mol.
- The molar mass, MMM, of a substance is the mass of one mole of it, measured in g/mol\text{g/mol}g/mol.
- The molar mass has the same numerical value as the relative mass of the particles.
- For atoms, the molar mass equals ArA_rAr, so 1 mol1\ \text{mol}1 mol of carbon atoms has a mass of 12 g12\ \text{g}12 g.
- For molecules and ionic compounds, the molar mass equals MrM_rMr, so 1 mol1\ \text{mol}1 mol of water has a mass of 18 g18\ \text{g}18 g.
- The key relationship is amount of substance (mol)=mass (g)molar mass (g/mol)\text{amount of substance (mol)} = \dfrac{\text{mass (g)}}{\text{molar mass (g/mol)}}amount of substance (mol)=molar mass (g/mol)mass (g).
- In symbols this is n=mMn = \dfrac{m}{M}n=Mm, which rearranges to m=n×Mm = n \times Mm=n×M and M=mnM = \dfrac{m}{n}M=nm.
- Finding MMM from a measured mass and amount tells you the ArA_rAr or MrM_rMr, because they have the same numerical value.
- The mass must be in grams, so convert a mass in kilograms by multiplying by 100010001000.
Amount and mass
- Calcium carbonate, CaCO3\text{CaCO}_3CaCO3, has Mr=40+12+(3×16)=100M_r = 40 + 12 + (3 \times 16) = 100Mr=40+12+(3×16)=100, so its molar mass is 100 g/mol100\ \text{g/mol}100 g/mol.
- Sodium hydroxide, NaOH\text{NaOH}NaOH, has Mr=23+16+1=40M_r = 23 + 16 + 1 = 40Mr=23+16+1=40, so 0.400 mol0.400\ \text{mol}0.400 mol of it has the mass below.
Identifying an element from its molar mass
- A sample of a metal containing 0.150 mol0.150\ \text{mol}0.150 mol of atoms has a mass of 3.60 g3.60\ \text{g}3.60 g.
- The metal has Ar=24A_r = 24Ar=24, so it is magnesium.
Particles from a mass
- A sample of carbon dioxide, Mr=44M_r = 44Mr=44, has a mass of 4.40 g4.40\ \text{g}4.40 g.
Molar gas volume
Molar gas volume
The volume occupied by one mole of any gas, taken as 24 dm324\ \text{dm}^324 dm3 at room temperature and pressure.
- At the same temperature and pressure, one mole of any gas takes up the same volume, whichever gas it is.
- This is because gas particles are very far apart, so the volume depends on the number of particles and hardly at all on their size.
- At room temperature and pressure, r.t.p., one mole of any gas is taken to occupy 24 dm324\ \text{dm}^324 dm3, which is 24 000 cm324\,000\ \text{cm}^324000 cm3.
- For a gas volume in dm3\text{dm}^3dm3, use amount (mol)=volume (dm3)24\text{amount (mol)} = \dfrac{\text{volume (dm}^3\text{)}}{24}amount (mol)=24volume (dm3).
- For a gas volume in cm3\text{cm}^3cm3, use amount (mol)=volume (cm3)24 000\text{amount (mol)} = \dfrac{\text{volume (cm}^3\text{)}}{24\,000}amount (mol)=24000volume (cm3).
- Rearranging gives volume (dm3)=amount (mol)×24\text{volume (dm}^3\text{)} = \text{amount (mol)} \times 24volume (dm3)=amount (mol)×24.
- These relationships apply only to gases, and only at r.t.p.
Gas volume calculations
- A sample of 0.350 mol0.350\ \text{mol}0.350 mol of oxygen at r.t.p. has the volume below.
- A volume of 600 cm3600\ \text{cm}^3600 cm3 of hydrogen at r.t.p. contains the amount below.
- A mass of 8.80 g8.80\ \text{g}8.80 g of carbon dioxide is 8.8044=0.200 mol\frac{8.80}{44} = 0.200\ \text{mol}448.80=0.200 mol, so its volume at r.t.p. is found next.
Reacting masses and gas volumes
- The coefficients in a balanced equation give the mole ratio of the reactants and products.
- N2+3H2→2NH3\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3N2+3H2→2NH3 means that 1 mol1\ \text{mol}1 mol of nitrogen reacts with 3 mol3\ \text{mol}3 mol of hydrogen to make 2 mol2\ \text{mol}2 mol of ammonia.
- A stoichiometric calculation uses this mole ratio to link the amount of one substance to the amount of another.
- Work through these steps in order:
- Convert the quantity you know into moles.
- Use the mole ratio from the equation to find the moles of the substance you want.
- Convert these moles into the mass, gas volume or solution volume that is asked for.
- For gases at r.t.p., the mole ratio is also the volume ratio, because equal amounts of gas occupy equal volumes.
- In H2+Cl2→2HCl\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}H2+Cl2→2HCl, 30 cm330\ \text{cm}^330 cm3 of hydrogen reacts with 30 cm330\ \text{cm}^330 cm3 of chlorine to make 60 cm360\ \text{cm}^360 cm3 of hydrogen chloride.
Mass of product from mass of reactant
2Mg(s)+O2(g)→2MgO(s) 2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)} 2Mg(s)+O2(g)→2MgO(s)- A mass of 6.00 g6.00\ \text{g}6.00 g of magnesium is 6.0024=0.250 mol\frac{6.00}{24} = 0.250\ \text{mol}246.00=0.250 mol.
- The mole ratio Mg:MgO\text{Mg}:\text{MgO}Mg:MgO is 2:22:22:2, which is 1:11:11:1, so 0.250 mol0.250\ \text{mol}0.250 mol of magnesium oxide forms.
Gas volume from mass of reactant
CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g) \text{CaCO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)} CaCO3(s)+2HCl(aq)→CaCl2(aq)+H2O(l)+CO2(g)- A mass of 5.00 g5.00\ \text{g}5.00 g of calcium carbonate is 5.00100=0.0500 mol\frac{5.00}{100} = 0.0500\ \text{mol}1005.00=0.0500 mol.
- The mole ratio CaCO3:CO2\text{CaCO}_3 : \text{CO}_2CaCO3:CO2 is 1:11:11:1, so 0.0500 mol0.0500\ \text{mol}0.0500 mol of carbon dioxide forms.
Mass of reactant from a gas volume
2H2O2(aq)→2H2O(l)+O2(g) 2\text{H}_2\text{O}_2\text{(aq)} \rightarrow 2\text{H}_2\text{O(l)} + \text{O}_2\text{(g)} 2H2O2(aq)→2H2O(l)+O2(g)- A volume of 480 cm3480\ \text{cm}^3480 cm3 of oxygen at r.t.p. is 48024 000=0.0200 mol\frac{480}{24\,000} = 0.0200\ \text{mol}24000480=0.0200 mol.
- The mole ratio H2O2:O2\text{H}_2\text{O}_2 : \text{O}_2H2O2:O2 is 2:12:12:1, so 0.0400 mol0.0400\ \text{mol}0.0400 mol of hydrogen peroxide decomposed.
- Hydrogen peroxide has Mr=(2×1)+(2×16)=34M_r = (2 \times 1) + (2 \times 16) = 34Mr=(2×1)+(2×16)=34.
Limiting reactants
Limiting reactant
The reactant that is completely used up first in a reaction, so it decides the maximum amount of product that can form.
Excess reactant
A reactant present in more than the amount needed to react completely, so some of it is left over when the reaction stops.
- Reactants are often mixed in amounts that do not match the mole ratio in the equation.
- The reaction stops when the limiting reactant has been used up, and the leftover reactant is said to be in excess.
- The amount of product always depends on the limiting reactant and never on the reactant in excess.
- Making cheese sandwiches works the same way, because each sandwich needs two slices of bread and one slice of cheese.
- With 101010 slices of bread and 888 slices of cheese you can make only 555 sandwiches, so the bread is limiting and 333 slices of cheese are left over.
- Find the limiting reactant by following these steps:
- Convert the quantity of each reactant into moles.
- Divide each amount by the coefficient of that reactant in the balanced equation.
- The reactant with the smaller answer is the limiting reactant.
- Use the moles of the limiting reactant and the mole ratio to find the amount of product.
Finding the limiting reactant
Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) \text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)} Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g)- A 2.40 g2.40\ \text{g}2.40 g piece of magnesium is 2.4024=0.100 mol\frac{2.40}{24} = 0.100\ \text{mol}242.40=0.100 mol, and it is added to acid containing 0.150 mol0.150\ \text{mol}0.150 mol of HCl\text{HCl}HCl.
- Dividing by the coefficients gives 0.1001=0.100\frac{0.100}{1} = 0.10010.100=0.100 for magnesium and 0.1502=0.0750\frac{0.150}{2} = 0.075020.150=0.0750 for hydrochloric acid.
- Hydrochloric acid gives the smaller value, so it is the limiting reactant and magnesium is in excess.
- The mole ratio HCl:H2\text{HCl}:\text{H}_2HCl:H2 is 2:12:12:1, so 0.0750 mol0.0750\ \text{mol}0.0750 mol of hydrogen forms.
Concentration calculations
Concentration
The quantity of solute dissolved in each cubic decimetre of solution, measured in g/dm3\text{g/dm}^3g/dm3 or mol/dm3\text{mol/dm}^3mol/dm3.
- Concentration links the quantity of dissolved solute to the volume of the solution.
- The concentration in moles is concentration (mol/dm3)=amount of solute (mol)volume of solution (dm3)\text{concentration (mol/dm}^3\text{)} = \dfrac{\text{amount of solute (mol)}}{\text{volume of solution (dm}^3\text{)}}concentration (mol/dm3)=volume of solution (dm3)amount of solute (mol).
- The concentration by mass is concentration (g/dm3)=mass of solute (g)volume of solution (dm3)\text{concentration (g/dm}^3\text{)} = \dfrac{\text{mass of solute (g)}}{\text{volume of solution (dm}^3\text{)}}concentration (g/dm3)=volume of solution (dm3)mass of solute (g).
- The volume must be in dm3\text{dm}^3dm3, so convert a volume in cm3\text{cm}^3cm3 by dividing by 100010001000.
- A volume of 250 cm3250\ \text{cm}^3250 cm3 is 2501000=0.250 dm3\frac{250}{1000} = 0.250\ \text{dm}^31000250=0.250 dm3.
- To change dm3\text{dm}^3dm3 back into cm3\text{cm}^3cm3, multiply by 100010001000.
- In symbols, c=nVc = \dfrac{n}{V}c=Vn rearranges to n=c×Vn = c \times Vn=c×V and V=ncV = \dfrac{n}{c}V=cn.
- To change a concentration in mol/dm3\text{mol/dm}^3mol/dm3 into g/dm3\text{g/dm}^3g/dm3, multiply by the molar mass of the solute.
- To change a concentration in g/dm3\text{g/dm}^3g/dm3 into mol/dm3\text{mol/dm}^3mol/dm3, divide by the molar mass of the solute.
Concentration of a prepared solution
- A mass of 5.85 g5.85\ \text{g}5.85 g of sodium chloride is dissolved in water and made up to 500 cm3500\ \text{cm}^3500 cm3, which is 0.500 dm30.500\ \text{dm}^30.500 dm3.
- Sodium chloride has Mr=23+35.5=58.5M_r = 23 + 35.5 = 58.5Mr=23+35.5=58.5, so the amount dissolved is 5.8558.5=0.100 mol\frac{5.85}{58.5} = 0.100\ \text{mol}58.55.85=0.100 mol.
Amount and volume from a concentration
- A 25.0 cm325.0\ \text{cm}^325.0 cm3 portion of 0.200 mol/dm30.200\ \text{mol/dm}^30.200 mol/dm3 sodium hydroxide has a volume of 0.0250 dm30.0250\ \text{dm}^30.0250 dm3.
- The volume of 0.250 mol/dm30.250\ \text{mol/dm}^30.250 mol/dm3 solution that contains 0.0500 mol0.0500\ \text{mol}0.0500 mol of solute is found by rearranging.
Titration calculations
Titre
The volume of solution added from the burette to reach the end-point of a titration.
Concordant titres
Titres from repeated titrations that agree closely with one another and are used to calculate the mean titre.
- A titration finds the exact volume of one solution that reacts with a measured volume of another.
- The titre is the volume added from the burette, found by subtracting the initial burette reading from the final reading.
- The titration is repeated, and the mean titre is calculated using only the concordant titres.
- From titres of 24.3024.3024.30, 23.6523.6523.65 and 23.75 cm323.75\ \text{cm}^323.75 cm3, the first value is left out because it does not agree with the others.
- The mean titre is 23.65+23.752=23.70 cm3\frac{23.65 + 23.75}{2} = 23.70\ \text{cm}^3223.65+23.75=23.70 cm3.
- With the mean titre, the calculation follows these steps:
- Use n=c×Vn = c \times Vn=c×V to find the moles in the solution whose concentration and volume are both known.
- Use the mole ratio from the balanced equation to find the moles of the other solute.
- Use c=nVc = \dfrac{n}{V}c=Vn to find an unknown concentration, or V=ncV = \dfrac{n}{c}V=cn to find an unknown volume.
- Multiply by the molar mass if the concentration is needed in g/dm3\text{g/dm}^3g/dm3.
- Every volume must be converted from cm3\text{cm}^3cm3 into dm3\text{dm}^3dm3 before it goes into n=c×Vn = c \times Vn=c×V.
Concentration of sodium hydroxide
- A 25.0 cm325.0\ \text{cm}^325.0 cm3 portion of sodium hydroxide is neutralised by a mean titre of 20.0 cm320.0\ \text{cm}^320.0 cm3 of 0.100 mol/dm30.100\ \text{mol/dm}^30.100 mol/dm3 nitric acid.
- The mole ratio is 1:11:11:1, so the sodium hydroxide also contains 0.00200 mol0.00200\ \text{mol}0.00200 mol.
- Sodium hydroxide has Mr=40M_r = 40Mr=40, so the concentration by mass is 0.0800×40=3.20 g/dm30.0800 \times 40 = 3.20\ \text{g/dm}^30.0800×40=3.20 g/dm3.
Titration with a 1 to 2 mole ratio
- A 25.0 cm325.0\ \text{cm}^325.0 cm3 portion of 0.200 mol/dm30.200\ \text{mol/dm}^30.200 mol/dm3 sodium hydroxide is neutralised by a mean titre of 20.0 cm320.0\ \text{cm}^320.0 cm3 of sulfuric acid.
- The mole ratio H2SO4:NaOH\text{H}_2\text{SO}_4 : \text{NaOH}H2SO4:NaOH is 1:21:21:2, so the acid contains 0.005002=0.00250 mol\frac{0.00500}{2} = 0.00250\ \text{mol}20.00500=0.00250 mol.
- Do not divide a gas volume in cm3\text{cm}^3cm3 by 242424, because 242424 is the molar gas volume in dm3\text{dm}^3dm3.
- Do not use the molar gas volume for solids, liquids or solutions, because it applies only to gases.
- Do not skip the mole ratio, because the moles of two substances are equal only when the ratio is 1:11:11:1.
- Do not include a rough or anomalous titre in the mean.
- Do not round numbers partway through a calculation, because early rounding can change the final answer.
Mole calculations in the exam
- Set out each calculation as a chain of labelled steps, because a mistake in one step can still let you earn the marks for the steps after it.
- Write the unit after every value, because a missing or wrong unit on the final answer usually loses that mark.
- Give the final answer to the same number of significant figures as the least precise data in the question, which is usually three.
- Write the mole ratio out in words, such as "1 mol1\ \text{mol}1 mol of CaCO3\text{CaCO}_3CaCO3 gives 1 mol1\ \text{mol}1 mol of CO2\text{CO}_2CO2", so you never forget to use it.
- How many particles are there in one mole of a substance?
- Which equation links amount of substance, mass and molar mass?
- What volume does one mole of any gas occupy at r.t.p.?
- How do you decide which reactant is the limiting reactant?
- How many moles of solute are in 50.0 cm350.0\ \text{cm}^350.0 cm3 of a 0.200 mol/dm30.200\ \text{mol/dm}^30.200 mol/dm3 solution?