Experimental Design Worked ExamplesIGCSE Chemistry 0620

(a) A has a scale along its length and a tap to let liquid out: a burette. B has a bulb and one mark: a volumetric pipette. C is a graduated cylinder on a base: a measuring cylinder. D is a barrel with a plunger that moves out as gas enters: a gas syringe.

(b)(i) A volumetric pipette delivers one fixed volume very accurately when filled to the mark: B.

(ii) A burette has a tap to add liquid drop by drop and a scale read to $0.05\text{ cm}^3$: A. A measuring cylinder (C) is far less accurate, and a pipette only delivers its one fixed volume.

(a) The solvent is the substance that dissolves the solute: (i) water. The solute is the substance that is dissolved: (ii) sodium chloride. Together they make a solution.

(b) A residue is the substance that remains after filtration (or evaporation, distillation…): here the sand left on the filter paper. The filtrate is the liquid or solution that has passed through the filter: the sodium chloride solution.

(c) A saturated solution contains the maximum concentration of solute that can dissolve in the solvent at a specified temperature. When the student evaporates water, the solution becomes saturated and crystals start to form.

(d) Sand is insoluble in water, so its particles stay as solid grains that are too large to pass through the pores of the filter paper.

$17.4\text{ cm}^3$ is not a standard pipette volume, and it is given to $0.1\text{ cm}^3$. A burette can deliver any volume and is read to $0.05\text{ cm}^3$, so B is the best choice.

A: a $50\text{ cm}^3$ measuring cylinder is usually graduated only in $1\text{ cm}^3$ divisions, so it cannot measure $17.4\text{ cm}^3$ precisely. C: a temperature rise needs a thermometer, not a balance. D: a gas volume needs a gas syringe (or an upturned measuring cylinder); a pipette measures one fixed volume of liquid.

(a) Each thermometer division is $1\,^{\circ}\text{C}$, so read to the nearest half division, $0.5\,^{\circ}\text{C}$, and record every reading to one decimal place. Start: $21.0\,^{\circ}\text{C}$. End: the top of the liquid is halfway between $27$ and $28$: $27.5\,^{\circ}\text{C}$.

(b) Read the bottom of the meniscus at eye level. It is on the $34$ line; the cylinder has $1\text{ cm}^3$ divisions, so record to the same half-division resolution: $34.0\text{ cm}^3$.

(c) $21$ has no decimal place, so it does not show the resolution of the thermometer ($0.5\,^{\circ}\text{C}$) and is not consistent with $27.5$. It should be $21.0$ — and a unit is needed in the table heading.

(a) Go up from $50\,^{\circ}\text{C}$ to the curve and across: $85$ g per $100$ g of water.

(b) Scale from $100$ g of water to $40.0$ g of water.

(c) At $20\,^{\circ}\text{C}$ the solubility is $32$ g per $100$ g of water, so $40.0$ g of water can hold only:

The rest comes out of solution as crystals.

(d) The liquid that passes through is the filtrate. It is still saturated at $20\,^{\circ}\text{C}$: crystals formed because it held the maximum amount it could at that temperature.

(a) Warm some distilled water in a beaker in a water-bath at $40\,^{\circ}\text{C}$, checked with a thermometer. Add potassium chloride a little at a time, stirring, until no more dissolves and some solid is left over (the solution is saturated).

Keep the mixture at $40\,^{\circ}\text{C}$ for a few minutes, then filter off the excess solid. Weigh an empty evaporating basin, pour in some filtrate and weigh again.

Heat the basin to evaporate all of the water; cool and weigh; reheat and reweigh until the mass is constant (to be sure it is dry). Calculate the mass of salt and the mass of water, then scale to $100$ g of water.

(b) Mass of dry salt and mass of water that held it:

Scale to $100$ g of water.

(a) A gas syringe.

(b) Advantage of X: carbon dioxide is slightly soluble in water, so in Y some gas dissolves and the volume is too low; a gas syringe also has a finer scale. Disadvantage of X: a gas syringe holds only about $100\text{ cm}^3$ (and the plunger can stick), so it cannot collect large volumes.

(c) No effect. The gas produced pushes air out of the flask into the syringe, but at the end the flask is full of carbon dioxide instead of air. The volume of air pushed out equals the volume of carbon dioxide left behind in the flask, so the total measured equals the volume of gas made.

(d) Keep the reactants apart until the flask is sealed: put the acid in a small test-tube standing inside the flask with the carbonate (or use a flask with two sections), put the bung in, then tip the flask to mix them.

(a) The runs at $1.0\text{ mol/dm}^3$ should agree closely. $47$ s is far from $35$ s and $33$ s, so $47$ s is anomalous. Leave it out of the mean.

(b) Repeating lets the student compare the results to check they agree and to spot anomalous results, which can then be left out of the mean.

(c) Rub the ribbon with sandpaper (emery paper) to remove the layer of magnesium oxide. The oxide layer reacts first and slows the start of the reaction differently each time, making the times too long and inconsistent.

(a) Water $= 40.0 -$ volume of acid.

(b) The acid is diluted from $10.0\text{ cm}^3$ to $40.0\text{ cm}^3$, a factor of $4$.

(c) The independent variable is what the student changes: the concentration of the acid (set by the volume of acid used). The dependent variable is what is measured: the rate (for example, the time for the magnesium to disappear).

(d) With the total volume fixed, the concentration of acid in each mixture is proportional to the volume of acid added, so concentration is the only thing that changes. A different total volume would also change the depth of liquid covering the magnesium and the amount of acid, so the results could not be compared.

(a) Use a polystyrene cup with a lid instead of the glass beaker: polystyrene is an insulator, so less heat is lost to the surroundings and the measured rise is closer to the true value.

Use a burette or a volumetric pipette instead of the measuring cylinder: these measure volume more accurately, so the amounts of acid and alkali are known more exactly.

(b) A water-bath keeps the temperature of the mixture constant. The experiment is measuring a temperature change, so a water-bath would reduce the change it is trying to measure.

(c) Stirring mixes the reactants so they react completely and the temperature is the same throughout the mixture; otherwise the thermometer may read a local hot or cold region.

Measure the water. Put a measured volume of distilled water, for example $50.0\text{ cm}^3$ from a burette or measuring cylinder, into a polystyrene cup (insulated to reduce heat gain from the surroundings).

Initial temperature. Measure the temperature of the water with a thermometer before adding the solid.

Add a fixed mass. Weigh a fixed mass, for example $5.00$ g, of ammonium chloride on a balance, add it to the water and stir until it has all dissolved. Record the lowest temperature reached.

Repeat with the other salt using the same volume of water, the same starting temperature and the same mass of potassium nitrate.

Process and conclude. Calculate temperature decrease $=$ initial temperature $-$ lowest temperature for each salt. The salt with the larger decrease gives the greater temperature decrease (the more endothermic dissolving per gram). Repeat each experiment and compare the results to check they agree.

(a) $M_r(\text{CaCO}_3) = 40 + 12 + 3(16) = 100$, so $n = 1.00 \div 100 = 0.0100$ mol, and the equation gives $0.0100$ mol of $\text{CO}_2$ ($M_r = 44$). This gas escapes, so the mass falls.

(b) The cotton wool lets the carbon dioxide escape but stops acid spray (droplets) leaving the flask, which would make the mass loss too large.

(c) At r.t.p. $0.0100$ mol of gas has a volume of $0.0100 \times 24\,000 = 240\text{ cm}^3$ — more than the $100\text{ cm}^3$ the syringe holds. A mass of $0.40$ g gives a volume that fits:

(d) $0.24$ g of Mg is $0.0100$ mol, giving $0.0100$ mol of $\text{H}_2$ ($M_r = 2$): a mass of only $0.020$ g. On a balance reading to $0.01$ g this change is just two divisions, so the uncertainty would be about $50\%$. Hydrogen is so light that its volume must be measured instead (it is still $240\text{ cm}^3$).

Safety: hydrogen is flammable, so keep flames away from the apparatus (and wear eye protection because the acid is an irritant).