10 minutes maximum

An IB Periodic Table and data booklet is required.

1. In an experiment to determine the empirical formula of magnesium oxide, a piece of magnesium ribbon of mass 0.50 ± 0.01 g was heated in a crucible to form magnesium oxide. The mass of the product was measured and the percentage composition of magnesium and oxygen in the final product was then calculated. What is the best way to reduce the systematic error in this experiment?
• A.   Using sandpaper to remove the layer of magnesium oxide on the surface of the magnesium ribbon before heating.
• B.   Using a greater mass of magnesium.
• C.   Repeating the experiment more times and finding the average of the results.
• D.   Using a balance which measures to 3 decimal places.

2. In an experiment to measure the enthalpy change of combustion of propan-1-ol, as shown in the diagram, which of the following contributes to the random errors in the result? • A.   The use of a glass beaker which does not conduct heat well.
• B.   The use of a balance which rounds measurements to 1 decimal place only.
• C.   The evaporation of alcohol from the burner.
• D.  The loss of heat to the surrounding air.

3. An experiment was set up to measure the potential difference of a voltaic cell, as shown in the diagram below, with a zinc half-cell and a copper half-cell. The literature value for the potential difference is 1.10V under standard conditions. The experiment was repeated 3 times using new strips of metals each time and potential differences measured was 1.37V, 1.39V and 1.38V. How would you describe the accuracy and precision of these results?

• A.   Not accurate and not precise
• B.   Accurate but not precise
• C.   Precise but not accurate
• D.   Accurate and precise

4. The literature value of the enthalpy change of the reaction

2KHCO3(s)  → K2CO3(s)+H2O(l)+CO2(g)

is +70.0 kJ mol-1. The experimentally measured value is +45.7 kJ mol-1. What is the percentage error in this experiment?

• A.   24.3%
• B.   34.7%
• C.   53.2%
• D.   65.3%

5. In an experiment to measure the enthalpy change of solution of sodium chloride, as shown in the diagram, which of the following statements about errors is correct? • A.   The random error in measuring temperature can be eliminated by using a more precise temperature probe, which measures to 2 decimal places.
• B.   Systematic errors can be reduced by using a more precise balance to measure the initial mass of the solute.
• C.   Random errors lead to imprecision in the results.
• D.   A systematic error in the experiment is the lack of repeats.
6. An investigation is carried out in order to study the effect of concentration of the reactant on the rate of hydrogen peroxide decomposition:

2H2O2(aq)  → 2H2O(l) + O2(g)

Which of the following is correct about the variables in this experiment?

 Independent Variable Dependent Variable Controlled Variable A. Volume of O2 given off in a period of time Concentration of H2O2 Temperature of the reaction mixture B. Concentration of H2O2 Temperature of the reaction mixture Volume of O2 given off in a period of time C. Volume of O2 given off in a period of time Temperature of the reaction mixture Concentration of H2O2 D. Concentration of H2O2 Volume of O2 given off in a period of time Temperature of the reaction mixture

Questions 7-9 are about a titration experiment, in which a standard solution of sodium hydrogen carbonate NaHCO3(aq) was used to find the concentration of an unknown hydrochloric acid HCl(aq) solution.

The set-up is shown in the diagram and methyl orange was used as an indicator.

A rough trial was carried out at the beginning and then three titrations were done carefully, adding the acid solution dropwise near the end point. The raw data are shown in the table below.

 Trial Rough 1 2 3 Initial burette reading / cm3 0.00 4.5 12.2 22.9 Final burette reading / cm3 26.50 29.5 37.3 48.1

The smallest division on the 50cm3 burette is 0.1 cm3. 7. What is the percentage uncertainty in the titre from trial 2?
• A.   0.100%
• B.   0.199%
• C.   0.398%
• D.   0.797%
8. Trials 1-3 produced concordant titre results and they were used to calculate the mean titre of the acid solution. Select the correct mean titre with its absolute uncertainty.
• A.  25.10± 0.05 cm3
• B.   25.1 ± 0.1 cm3
• C.   25.10 ± 0.10 cm3
• D.   25.1 ± 0.3 cm3

9. The concentration of the sodium hydrogencarbonate standard solution NaHCO3(aq) was 0.096 mol dm-3, and its volume was measured using a 25.00 ± 0.06 cm3 pipette.

When calculating the concentration of the acid solution HCl(aq), how should its uncertainties be calculated?

• A.  The absolute uncertainty in the HCl(aq) concentration is equal to the sum of the percentage uncertainties in the volume of the NaHCO3(aq) and in the volume of the acid titre.
• B.   The percentage uncertainty in the HCl(aq) concentration is equal to the sum of the percentage uncertainties in the volume of the NaHCO3(aq) and in the volume of the acid titre.
• C.   The absolute uncertainty in the HCl(aq) concentration is equal to the product of the percentage uncertainties in the volume of the NaHCO3(aq) and in the volume of the acid titre.
• D.   The percentage uncertainty in the HCl(aq) concentration is equal to the product of the percentage uncertainties in the volume of the NaHCO3(aq) and in the volume of the acid titre.
10. Which of the following graphs correctly represent the relationship between pressure P and volume V for an ideal gas when the temperature of the gas and the mole of the gas remain constant? • A. I and II only
• B. I and III only
• C. II and III only
• D. I, II and III