IB Chemistry – Data-Based Questions (Paper 1B)

A student titrates an acid into an alkali. The initial burette reading is $0.20\,\text{cm}^3$ and the final reading is $23.60\,\text{cm}^3$, each read to an uncertainty of $\pm0.05\,\text{cm}^3$.(a) Calculate the titre (volume added) and its absolute uncertainty.(b) Calculate the percentage uncertainty in the titre.

A digital balance reads to $\pm0.005\,\text{g}$. A student finds the mass of a sample by subtracting an empty container’s mass ($12.40\,\text{g}$) from the full container’s mass ($14.75\,\text{g}$).(a) State the mass of the sample and its absolute uncertainty.(b) Calculate the percentage uncertainty in this mass.

In a calorimetry experiment, the initial temperature of the water is $21.0\,^{\circ}\text{C}$ and the final temperature is $29.4\,^{\circ}\text{C}$, each read from a thermometer with an uncertainty of $\pm0.1\,^{\circ}\text{C}$.Calculate the percentage uncertainty in the temperature change, $\Delta T$.

A calculated quantity $k$ is found using $k=\dfrac{x}{yz}$, where $x$ has a percentage uncertainty of $2\%$, $y$ has $1\%$ and $z$ has $3\%$. The calculated value is $k=15.0$.State $k$ together with its absolute uncertainty, showing your method.

In an enthalpy of combustion experiment, burning ethanol ($\text{C}_2\text{H}_5\text{OH}$, $M=46.08\,\text{g mol}^{-1}$) heats $150\,\text{g}$ of water (specific heat capacity $4.18\,\text{J g}^{-1}\,^{\circ}\text{C}^{-1}$) from $21.0\,^{\circ}\text{C}$ to $29.4\,^{\circ}\text{C}$. The mass of ethanol burned is $0.18\,\text{g}$.(a) Calculate the heat energy transferred to the water.(b) Calculate the experimental enthalpy of combustion of ethanol, in $\text{kJ mol}^{-1}$.

A gas is collected from a reaction and its volume recorded at intervals: $t=0\,\text{s}, V=0\,\text{cm}^3$; $t=10\,\text{s}, V=18\,\text{cm}^3$; $t=20\,\text{s}, V=31\,\text{cm}^3$; $t=30\,\text{s}, V=40\,\text{cm}^3$; $t=40\,\text{s}, V=45\,\text{cm}^3$ (the reaction is essentially complete by $40\,\text{s}$).Use the first two data points to estimate the initial rate of reaction, stating units.

A calibration graph of absorbance against concentration for a coloured solution is a straight line through the origin, passing through the point $(0.020\,\text{mol dm}^{-3}, 0.42)$.A sample solution has an absorbance of $0.29$. Calculate its concentration.

In a titration curve for a weak acid with $\text{NaOH}(aq)$, the pH at the half-equivalence point is $4.76$.(a) State the value of $\text{p}K_a$ for the weak acid, with a reason.(b) Calculate $K_a$.

A sample of magnesium of mass $0.024\,\text{g}$ ($A_r=24.31$) reacts completely with excess dilute hydrochloric acid: $\text{Mg}+2\text{HCl}\rightarrow\text{MgCl}_2+\text{H}_2$. The volume of hydrogen gas collected at room temperature and pressure is $23.7\,\text{cm}^3$.Use this data to calculate an experimental value for the molar volume of a gas at room temperature and pressure.

A graph of $\ln[\text{A}]$ against time for a first-order reaction is a straight line. Two points read from the graph are $(0\,\text{s}, -1.2)$ and $(100\,\text{s}, -3.5)$.Use the gradient of this graph to calculate the rate constant $k$ for the reaction, stating its unit.

In the enthalpy of combustion experiment (Section B, Q1), identify one source of systematic error and one source of random error, and explain the effect of the systematic error on the calculated enthalpy value.

The literature value for the enthalpy of combustion of ethanol is $-1367\,\text{kJ mol}^{-1}$. Using the experimental value from Section B, Q1(b), calculate the percentage error in the experimental result.

Suggest one modification to the calorimetry experiment in Section B, Q1 that would reduce the systematic error identified above, and explain how it does so.