IB Chemistry Chemical inquiry worked solutions

A student compares endpoint times at different initial concentrations. Which change best isolates the effect of concentration? A. change the temperature with each concentration

B. keep total volume and the small detected product concentration fixed

C. choose a different endpoint colour for each run

D. use the shortest result only

A reported molar concentration has unit \mathrm{mol\,dm^{-3}}. What does \mathrm{dm^{-3}} contribute to the meaning? A. per cubic decimetre

B. a negative concentration

C. a cubed negative length

D. a negative ion charge

A spreadsheet gives R^{2}=0.997 for a straight-line fit of two measured variables. Which conclusion is justified?

A. The gradient must be positive.

B. The line accounts for most of the variation in the dependent variable.

C. The independent variable has been proved to cause the change.

D. Every measurement has an uncertainty of 0.3\%.

A calibration line for a laboratory gauge is supported by measurements between 4.0 and 14.0\,\text{mm}. An unknown reading corresponds to 21.0\,\text{mm} on that line. Which is the strongest next step before reporting the result?

A. Report it because a straight line remains valid for every value.

B. Add repeated measurements at 9.0\,\text{mm} only.

C. Extend the calibration with known standards near 21.0\,\text{mm}.

D. Delete the largest existing calibration point.

During repeated drop-time measurements, one value is much larger than the rest. The record states that the timer failed to stop and continued running after that drop had landed. Which treatment is justified?

A. Exclude the value and document the recorded timing failure.

B. Keep it because every reading is automatically valid.

C. Exclude the value only because it is furthest from the mean.

D. Replace it with the mean and present it as a measured value.

Two fuel tests report energy releases per gram: U, (21.6\pm0.8)\,\text{kJ g}^{-1}; V, (22.4\pm1.0)\,\text{kJ g}^{-1}. Which conclusion is best supported?

A. V is conclusively better because its central value is larger.

B. The reported intervals overlap, so the tests do not establish a clear ranking.

C. U releases more energy per gram because its uncertainty is smaller.

D. V releases more energy because its upper limit is higher.

A laboratory needs the dissolution enthalpy of dry sodium ethanoate. Its molar mass is supplied. An insulated cup, lid, balance and temperature probe are available. The salt must dissolve completely.

(a) Outline a calorimetry method, including the mass and temperature measurements needed.   [4]

(b) Explain how the measurements give the molar enthalpy, assuming a known solution specific heat capacity and negligible cup heat capacity.   [2]

In an exothermic dissolution, a probe is held beside the freshly added crystals without stirring. It briefly reports a higher temperature than a second probe in the bulk liquid. A student uses this brief local reading to calculate the reaction enthalpy.

(a) Explain the error in this choice and its likely effect on the calculated enthalpy.   [2]

(b) Suggest two changes that would give a representative solution temperature.   [2]

An analyser reports [\mathrm Q]=(0.0468\pm0.0004)\,\mathrm{mol\,dm^{-3}} and then (0.0312\pm0.0004)\,\mathrm{mol\,dm^{-3}} after (52.0\pm0.5)\,\mathrm{s}. Use maximum absolute uncertainty addition for subtraction and percentage uncertainty addition for division.

(a) Calculate the mean disappearance rate.   [2]

(b) Determine the percentage uncertainty in this rate.   [2]

(c) Express the rate with an appropriate absolute uncertainty.   [1]

A sensor detects a fixed, small product concentration in each trial. Reaction conditions other than initial reagent concentration are held fixed, and the concentration change before detection is negligible relative to each initial reagent concentration. Times to detection are:\begin{array}{cc}c_0/\mathrm{mol\,dm^{-3}} & t/\mathrm s\\\hline 0.0400 & 120\\0.0800 & 60\\0.120 & 40\end{array}

(a) Calculate reciprocal times and state the concentration dependence supported by these data.   [2]

(b) Explain why reciprocal time is a comparable rate proxy here.   [2]

(c) Suggest how to test the trend more convincingly.   [1]

Endpoint times at one concentration are 54.1, 54.3, 79.8 and 54.2 s. All runs were completed. The notebook identifies no timing fault, but records incomplete mixing during the 79.8 s run. (a) Explain why replacing 79.8 s with 54.2 s and presenting four measured readings would be unacceptable.   [1] (b) State two actions needed before deciding whether the unusual run is valid for the intended method.   [2] (c) State one likely improvement to the collection procedure.   [1]

A database study compares published solubilities. Source A states grams of anhydrous solute per 100 g water at 298 K; source B states grams of hydrated solid per 100 g solution at 308 K. A simulation gives a smooth curve generated from a fitted equation using source A. (a) Identify two reasons the published numbers cannot be pooled unchanged.   [2] (b) Explain whether the simulated curve independently confirms source A.   [1] (c) State one record needed to make the secondary-data selection traceable.   [1]

Initial-rate measurements for a single-reactant model give:\begin{array}{cc}[\mathrm Q]_0/\mathrm{mol\,dm^{-3}} & r_0/\mathrm{mol\,dm^{-3}\,s^{-1}}\\\hline .040 & 1.6\times10^{-3}\\.080 & 3.2\times10^{-3}\\.120 & 4.8\times10^{-3}\end{array}

(a) Plot the three points with labelled axes and draw the expected best-fit line.   [2]

(b) Deduce the order and rate law from the graph.   [1]

(c) Calculate k, with units.   [2]

(d) Predict the initial rate for [\mathrm Q]_0=0.100\,\mathrm{mol\,dm^{-3}}.   [1]

An approved gas-density method weighs the same dry vessel before and after filling at fixed temperature and pressure. Empty mass =(65.214\pm0.002)\,\mathrm g; filled mass =(65.286\pm0.002)\,\mathrm g; gas volume =(40.0\pm0.4)\,\mathrm{cm^3}. Assume no leakage or retained liquid. Use maximum absolute bounds for subtraction and add relative bounds for division. (a) Calculate gas mass and its absolute uncertainty.   [2] (b) Calculate density in \mathrm{g\,dm^{-3}} and its percentage uncertainty.   [2] (c) Give density with an absolute uncertainty and judge compatibility with a supplied reference value of 1.90\,\mathrm{g\,dm^{-3}}, whose uncertainty is negligible here and whose conditions match.   [2] (d) Suggest one feasible measurement change that reduces the largest relative contribution without assuming that repeats remove bias.   [1]

A blank-correctable assay gives A=0.750c+0.020 for c in \mathrm{mmol\,dm^{-3}}, validated from 0 to 0.600\,\mathrm{mmol\,dm^{-3}}. A 10.00\,\mathrm{cm^3} sample is diluted to 50.00\,\mathrm{cm^3}. Its diluted absorbance is 0.320\pm0.015; take line coefficients and volume uncertainties as negligible for this exercise. (a) Calculate the original concentration.   [2] (b) Obtain its absolute uncertainty from the stated absorbance bound.   [2] (c) A high-concentration standard at 1.00\,\mathrm{mmol\,dm^{-3}} gives A=0.610\pm0.015. Explain what this suggests about extrapolating the line.   [2]