Concentration of Solutions

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Concentration of Solutions — Free MYP4 Chemistry Practice Questions

1QuestionCalculating moles from concentration and volumeConcept Practice

3 marks~5 minCriterion A

When 50 cm³ of 1.0 mol/dm³ hydrochloric acid reacts with excess calcium carbonate chips, the mass of CO₂ produced increases rapidly at first, then more slowly after 2 minutes.

Calculate the initial number of moles of HCl present. [1]

Explain how the number of moles of HCl changes during the reaction and what this means for the concentration of HCl in solution. [1]

Using collision theory, explain why the rate of reaction decreases as the reaction proceeds. [1]

Solutions

2QuestionCalculating moles from concentration and volumeAssessment Practice

6 marks~9 minCriterion C

A student titrates

25.0 \ \text{cm}^3

of hydrochloric acid (

\text{HCl}

) against

0.100 \ \text{mol dm}^{-3}

sodium hydroxide (

\text{NaOH}

) solution, using phenolphthalein as the indicator. Five runs are recorded below.

Run	1	2	3	4	5
Initial reading (cm³)	0.00	0.10	0.20	0.00	0.15
Final reading (cm³)	26.50	26.60	27.80	26.40	26.55
Titre volume (cm³)	26.50	26.50	27.60	26.40	26.40

Identify the anomalous titre value and justify your choice. [1]

Calculate the mean titre volume in

\text{cm}^3

, excluding the anomalous value. Show your working. [2]

Using your mean titre volume, calculate the number of moles of

\text{NaOH}

that reacted. [2]

Deduce the number of moles of

\text{HCl}

present in the

25.0 \ \text{cm}^3

sample, given that the neutralisation reaction is:

\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}

Justify your reasoning using the mole ratio. [1]

Solutions

3QuestionDilution concept and simple dilution calculationsAssessment Practice

8 marks~12 minCriterion D

A chemical factory discharges sulfuric acid waste at a concentration of

0.050 \ \text{mol dm}^{-3}

and a flow rate of

2.0 \ \text{m}^3 \text{h}^{-1}

into a river. The river's upstream flow rate is

500 \ \text{m}^3 \text{h}^{-1}

and initially contains no acid. Assume perfect mixing and no chemical reactions unless stated otherwise.

Calculate the concentration of sulfuric acid in the river downstream of the discharge point. [2]

Deduce whether the downstream concentration would be higher or lower than your calculated value during a period of heavy rainfall. Justify your answer with reference to the dilution formula. [2]

Evaluate the suitability of the model

C_1V_1 = C_2V_2

for predicting actual acid concentrations in this river, discussing the assumption of no chemical reactions and one further limitation. [4]

Solutions

4QuestionCalculating moles from concentration and volumeAssessment Practice

2 marks~3 minCriterion B

A student prepares four solutions by diluting a

2.0 \ \text{mol dm}^{-3}

NaCl stock solution to a total volume of

100 \ \text{cm}^3

each time.

Volume of stock solution ( $\text{cm}^3$ )	10	20	30	40
Final concentration ( $\text{mol dm}^{-3}$ )	0.20	0.40	0.60	0.80

Deduce the final concentration when

50 \ \text{cm}^3

of stock solution is used. [1]

Explain why this relationship between volume of stock solution and final concentration only holds when the total final volume remains constant. [1]

Solutions

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