A standard aqueous solution of chloroacetic acid (${CH}_{2}ClCOOH$) is prepared by dissolving $1180mg$ of the pure solid in distilled water and diluting to a total volume of exactly $150{cm}^3$. At $298K$, the $pH$ of this solution is measured to be $2.03$.

Calculate the value of $K_a$ for chloroacetic acid at this temperature. Give your answer to three significant figures. State the units of $K_a$.