Chemistry 5070 · O Level · Acid–base titrations

Acid–base titrations — practice question

A student receives dilute sulfuric acid labelled L. The concentration of L is found by titration. To measure and move $25.0\,\text{cm}^3$ of L into apparatus B, the student uses apparatus A. The diagrams display apparatus A and B. The student dilutes the $25.0\,\text{cm}^3$ portion of L to $500\,\text{cm}^3$ with distilled water in apparatus B. The resulting solution is M. The student then transfers $25.0\,\text{cm}^3$ of M into a conical flask and adds two drops of methyl orange indicator. Solution R is $0.100\,\text{mol dm}^{-3}$ aqueous sodium hydroxide, $\text{NaOH(aq)}$. R is delivered from a burette into the conical flask until the indicator alters colour and the end-point is reached.
(a(i))[1]

Name apparatus A.

(a(ii))[1]

Name the other piece of apparatus that should be used with A to measure $25.0\,\text{cm}^3$ of L.

(a(iii))[1]

Why is apparatus A used instead of a measuring cylinder?

(b)[1]

Name apparatus B.

(c)[1]

Which liquid is used to wash out the burette immediately before it is filled with R?

(d)[1]

What is the colour change of the methyl orange indicator at the end-point? The colour changes from ................. to ................. .

(e(i))[4]

Use the diagrams to fill in the table.

(e(ii))[1]

Why was it unnecessary for the student to do a third titration?

(f)[1]

Calculate the number of moles of $\text{NaOH}$ in the average volume of $0.100\,\text{mol dm}^{-3}\ \text{NaOH}$ used in the titration. Give your answer to three significant figures.

(g)[1]

Dilute sulfuric acid reacts with aqueous sodium hydroxide according to the equation shown. $\text{H}_2\text{SO}_4 + 2\text{NaOH} \rightarrow \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O}$ Calculate the number of moles of $\text{H}_2\text{SO}_4$ in $25.0\,\text{cm}^3$ of M.

(h)[1]

Calculate the number of moles of $\text{H}_2\text{SO}_4$ in $500\,\text{cm}^3$ of M.

(i)[1]

Calculate the number of moles of $\text{H}_2\text{SO}_4$ in $25.0\,\text{cm}^3$ of L.

(j)[1]

Calculate the concentration of $\text{H}_2\text{SO}_4$ in L.

(k(i))[2]

State and explain whether the second student’s titration volume of $\text{NaOH(aq)}$ would be smaller, larger or unchanged compared with the first student’s titration volume of $\text{NaOH(aq)}$.

(k(ii))[2]

State and explain whether the third student’s titration volume of $\text{NaOH(aq)}$ would be smaller, larger or unchanged compared with the first student’s titration volume of $\text{NaOH(aq)}$.

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