:"Perform the final pH calculation."}],"highlights":[{"text":"2 decimal places","location":"specification"}]}]}],"generatedAt":"2026-06-11T09:32:40.266Z","modelVersion":"gemini-3.5-flash","explanationVersion":"v2"},"options":[]},{"id":"57b4d3a7-8482-4283-b2c3-afbd95250f59","label":"d","content":"A student is provided with hydrated nickel(II) sulfate, \$\\text{NiSO}_4 \\cdot 6\\text{H}_2\\text{O}\$.\n\nThe student needs to prepare a standard solution of \$\\text{NiSO}_4 \\cdot 6\\text{H}_2\\text{O}\$ with a concentration of \$0.150\\text{ mol dm}^{-3}\$. Describe how the student would prepare \$250.0\\text{ cm}^3\$ of this solution, giving quantities, apparatus and method.\n\n[Use molar masses: \$\\text{Ni} = 58.7\\text{ g mol}^{-1}\$, \$\\text{S} = 32.1\\text{ g mol}^{-1}\$, \$\\text{O} = 16.0\\text{ g mol}^{-1}\$, \$\\text{H} = 1.0\\text{ g mol}^{-1}\$]","markscheme":"- Calculates molar mass of $\\text{NiSO}_4 \\cdot 6\\text{H}_2\\text{O} = 58.7 + 32.1 + (4 \\times 16.0) + (6 \\times 18.0) = 262.8\\text{ g mol}^{-1}$ B1\n- Calculates mass required = $0.150 \\times \\frac{250.0}{1000} \\times 262.8 = 9.86\\text{ g}$ B1\n- Weighs the solid and dissolves in less than $250\\text{ cm}^3$ of distilled water in a beaker, stirring with a glass rod M1\n- Transfers the solution quantitatively to a $250.0\\text{ cm}^3$ volumetric flask M1\n- Rinses the beaker and glass rod, adding the washings to the flask M1\n- Fills the flask with distilled water up to the graduation mark (bottom of meniscus on the mark), stoppers, and inverts several times to mix A1","marks":6,"order":3,"markingRubric":null,"explanation":{"methods":[{"id":"standard_preparation","label":"Standard Laboratory Procedure","steps":[{"type":"text","order":0,"title":"Calculate the molar mass","content":"To prepare the standard solution, we first need to calculate the molar mass ($M$) of the hydrated salt, $\\text{NiSO}_4 \\cdot 6\\text{H}_2\\text{O}$, using the provided relative atomic masses:\n\n$$\\begin{aligned} M(\\text{NiSO}_4 \\cdot 6\\text{H}_2\\text{O}) &= 58.7 + 32.1 + (4 \\times 16.0) + 6 \\times [2(1.0) + 16.0] \\\\ &= 58.7 + 32.1 + 64.0 + 6 \\times 18.0 \\\\ &= 262.8\\text{ g mol}^{-1} \\end{aligned}$$\n\n*This calculation earns **1 mark**.*","highlights":[{"text":"Ni = 58.7 g mol-1","location":"specification"},{"text":"S = 32.1 g mol-1","location":"specification"},{"text":"O = 16.0 g mol-1","location":"specification"},{"text":"H = 1.0 g mol-1","location":"specification"}]},{"type":"text","order":1,"title":"Calculate the required mass","content":"Next, we calculate the required mass ($m$) of $\\text{NiSO}_4 \\cdot 6\\text{H}_2\\text{O}$ to make $250.0\\text{ cm}^3$ of a $0.150\\text{ mol dm}^{-3}$ solution.\n\nFirst, find the amount in moles ($n$):\n$$n = c \\times V = 0.150\\text{ mol dm}^{-3} \\times \\frac{250.0}{1000}\\text{ dm}^3 = 0.0375\\text{ mol}$$\n\nNow, calculate the mass ($m$):\n$$m = n \\times M = 0.0375\\text{ mol} \\times 262.8\\text{ g mol}^{-1} = 9.855\\text{ g} \\approx 9.86\\text{ g}$$\n\n*This calculation earns **1 mark**.*","highlights":[{"text":"0.150 mol dm-3","location":"specification"},{"text":"250.0 cm3","location":"specification"}]},{"type":"text","order":2,"title":"Weigh and dissolve","content":"Now, we write down the practical steps to prepare this solution:\n\n1. Weigh exactly $9.86\\text{ g}$ of hydrated nickel(II) sulfate using a weighing boat and a balance (precision of at least 2 decimal places).\n2. Transfer the solid to a clean beaker. Re-weigh the empty boat to account for any residue (weighing-by-difference technique).\n3. Add about $100\\text{ cm}^3$ (less than $250\\text{ cm}^3$) of distilled/deionised water to the beaker.\n4. Stir the mixture thoroughly with a glass rod until the solid completely dissolves.\n\n*Dissolving in less than the final volume in a beaker with a glass rod earns **1 mark**.*"},{"type":"text","order":3,"title":"Quantitative transfer","content":"Once dissolved, we transfer the chemical mixt

Acids, bases and buffers

Question 32

Acids, bases and buffers Questions

Acids, bases and buffers

Question 32

Acids, bases and buffers Questions