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The particles of a gas are considered to be in constant random motion. For a fixed mass of gas in a container, the particles collide with the container walls and it is these collisions that are responsible for the pressure exerted by the gas.
The more frequent the collisions, the greater the pressure. If the temperature is kept constant, the average kinetic energy of the gas remains constant. A temperature increase means an increase in this average kinetic energy.
(a) (i) Predict how the pressure of a fixed mass of gas held at a constant temperature varies as the volume of the gas decreases. Explain this prediction in terms of the frequency of the gas collisions with the container walls.
Predict how the pressure will change .................................................................
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Explanation .............................................................................................................
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(ii) Display your prediction in the form of a sketch graph, labelling clearly the axes.
(iii) Sketch a second graph showing how your projected relationship will change if the temperature of the fixed mass of gas is increased. Label this second graph clearly.
[3]
(b) If you were to carry out an experiment to investigate how the pressure of a fixed mass of gas varies as the volume decreases, name
(i) the independent variable ............................................................
(ii) the dependent variable ...............................................................
[2]
512
538
558
You are to plan an experiment to investigate the decomposition of copper(II) carbonate on heating and hence decide which of the two equations represents the actual decomposition. The volume of gas liberated per mole of copper(II) carbonate depends on which equation is correct and this should be used as the basis of the plan.
(a) Draw a diagram of the apparatus and experimental setup you would use in the experiment. Your apparatus should use only standard items found in a school or college laboratory and should show clearly
(i) how the copper(II) carbonate will be heated,
(ii) how the volume of the gas evolved will be collected and its volume measured.
Label each piece of apparatus used, indicating its size or capacity. [3]
(b) Using the apparatus shown in (a), design a laboratory experiment which will enable you to determine the way in which copper(II) carbonate decomposes on heating.
In addition to the standard apparatus present in a laboratory, you are provided with the following material.
a sample of copper(II) carbonate
Give a step-by-step description of how you would carry out the experiment by
(i) calculating the volume of gas evolved by decomposing 1 mole of copper(II) carbonate according to equation 2.1.
(ii) calculating the volume of gas evolved by decomposing 1 mole of copper(II) carbonate according to equation 2.2.
(iii) using these results to calculate a mass of copper(II) carbonate that would give volumes of gas that could be collected using the apparatus proposed, under either decomposition.
(iv) stating how you would ensure that the decomposition is complete.
(v) stating how you would use your results to reach a conclusion.
[A_{\text{r}}: C, 12.0; O, 16.0; Cu, 63.5; the molar gas volume at 25^{\circ}C is 24.0 \text{dm}^{3}] [5]
(c) State one hazard that must be considered when planning the experiment and describe a precaution that should be taken to keep risks to a minimum. [2]
515
582
516
(a) The concentration of aqueous sulfuric acid can be determined by measuring the density of the solution. In order to establish a calibration graph, various solutions of known percentage composition by mass were set up using very accurate apparatus. The preparation of aqueous sulfuric acid is exothermic. Once each of the solutions had cooled, the actual density was determined using a hydrometer. The table below shows some of the calculations involved, together with the measured densities. To calculate the actual volumes of sulfuric acid and water mixed, the mass was used in conjunction with the appropriate density. The density of pure water is 0.997 g cm$^{-3}$ and the density of pure sulfuric acid is 1.826 g cm$^{-3}$. All measurements were carried out at 25$^{\circ}$C.
(b) On the grid below, using appropriate scales and labelled axes, plot two curves to show the variation of both the calculated densities and the measured densities against the percentage of sulfuric acid. Label each curve.
(c) (i) Using your curves, deduce the difference between the calculated and measured densities for the 45\% by mass of sulfuric acid mixture. State which of the two has the larger density. (ii) By considering the molecular structures of the two liquids, suggest an explanation for the difference.
(d) You have 100.000 g of a 60 \% by mass sulfuric acid mixture of a calculated density of 1.370 g cm$^{-3}$. Calculate the mass of water you would need to add to the 100.000 g in order to give a final calculated density of 1.154 g cm$^{-3}$.
(e) Attempting to determine the measured density of aqueous sulfuric acid in a school/college laboratory would involve using less accurate apparatus than in (a). Calculate the errors that could arise when measuring both the mass and volume of 100 g of sulfuric acid using a balance accurate to the nearest 0.01 g and a 100 cm$^{3}$ measuring cylinder accurate to the nearest 0.25 cm$^{3}$.
mass error
volume error
581
572
515
