Chemistry 9701/51 — October/November 2016

Cambridge A-Level · Planning, Analysis and Evaluation · worked solutions for every part, with the mark scheme

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Topics Analysis, Conclusions and Evaluation · Planning

Q1Analysis, Conclusions and EvaluationPlanningFree sample

When hydrated barium chloride, $\text{BaCl}_2 \cdot x\text{H}_2\text{O}$ , dissolves in water, $\text{Ba}^{2+}(\text{aq})$ and $\text{Cl}^-(\text{aq})$ ions are formed.

The concentration of chloride ions in solution can be determined by titration with aqueous silver nitrate of known concentration.

\text{Ag}^+(\text{aq}) + \text{Cl}^-(\text{aq}) \rightarrow \text{AgCl}(\text{s})

The indicator for the reaction is aqueous potassium chromate(VI), $\text{K}_2\text{CrO}_4(\text{aq})$ . At the endpoint of the titration, it forms a red precipitate in the presence of excess silver ions.

(a)

The solubilities, in $\text{g dm}^{-3}$ , of different ionic compounds at $20\text{ }^\circ\text{C}$ are given in the table below.

	anion
cation	$\text{Cl}^-$	$\text{CrO}_4^{2-}$	$\text{SO}_4^{2-}$
$\text{Ag}^+$	0.0019	0.022	293
$\text{Ba}^{2+}$	358	0.0028	0.00245

With reference to these data, where relevant, answer the following questions.

(i)

Name the red precipitate and give an equation for its formation.

name:

equation:

(ii)

Sulfuric acid must be added to the solution to prevent the $\text{Ba}^{2+}(\text{aq})$ ions from interfering with the action of the potassium chromate(VI) indicator.

How would $\text{Ba}^{2+}(\text{aq})$ ions interfere with the action of this indicator?

(iii)

How does the addition of sulfuric acid prevent $\text{Ba}^{2+}(\text{aq})$ ions from interfering with the action of this indicator?

(b)

In an initial rough titration, excess silver nitrate solution is added so that the endpoint is exceeded.

Draw a sketch graph to show how the mass of silver chloride varies with the volume of silver nitrate added.

Label both axes.

Fig. 1.1

(c)

You are to plan a titration experiment to determine the value of $x$ in $\text{BaCl}_2 \cdot x\text{H}_2\text{O}$ .

You are provided with the following materials.

$3.00\text{ g}$ of hydrated barium chloride, $\text{BaCl}_2 \cdot x\text{H}_2\text{O}$
$0.050\text{ mol dm}^{-3}$ aqueous silver nitrate
$1.0\text{ mol dm}^{-3}$ potassium chromate(VI) solution
$1.0\text{ mol dm}^{-3}$ sulfuric acid

(i)

Name three pieces of volumetric apparatus you would use, with their capacities in $\text{cm}^3$ .

(ii)

Describe how you would make a solution of barium chloride that is suitable for use in your titration.

(iii)

A known volume of barium chloride solution is transferred to a conical flask.

In what order should the other three solutions then be added to the flask?

first:

second:

third:

(iv)

How would you ensure that your titration result is reliable?

(v)

In another experiment, a student dissolved $3.13\text{ g}$ of hydrated barium chloride, $\text{BaCl}_2 \cdot x\text{H}_2\text{O}$ , in distilled water to give $1.00\text{ dm}^3$ of solution.

It was calculated that the concentration of $\text{Ba}^{2+}(\text{aq})$ ions was $0.0128\text{ mol dm}^{-3}$ .

Determine the value of $x$ in $\text{BaCl}_2 \cdot x\text{H}_2\text{O}$ .

[ $A_r$ : Ba, 137.3; Cl, 35.5; H, 1.0; O, 16.0]

$x =$

(d)

The following information gives some of the hazards associated with the chemicals used in the procedure.

Chemical	Hazard Information
Barium chloride	Solid barium chloride is classified as toxic. Solutions equal to or more concentrated than $0.4\text{ mol dm}^{-3}$ are classified as moderate hazard and are harmful if swallowed. Solutions less concentrated than $0.4\text{ mol dm}^{-3}$ are classified as non-hazardous.
Potassium chromate(VI)	All solutions more concentrated than $0.9\text{ mol dm}^{-3}$ are classified as health hazard. They may cause skin, eye and respiratory irritation.
Silver nitrate	Solutions equal to or more concentrated than $0.18\text{ mol dm}^{-3}$ are classified as corrosive. Solutions equal to or more concentrated than $0.06\text{ mol dm}^{-3}$ but less than $0.18\text{ mol dm}^{-3}$ are classified as moderate hazard and cause skin and eye irritation. Solutions less concentrated than $0.06\text{ mol dm}^{-3}$ are classified as non-hazardous.

Identify one hazard that must be considered when planning the experiment and describe a precaution, other than eye protection, that should be taken to keep risks from this hazard to a minimum.

hazard:

precaution:

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