Collecting and processing data practice questions.

A reaction produces 4.00 g of product from a theoretical maximum of 5.00 g. Calculate the percentage yield.

• 80.0%
• 125%
• 20.0%
• 1.25%

Answer: 80.0%

Percentage yield = (actual/theoretical) × 100 = (4.00/5.00) × 100 = 80.0%. The 125% distractor comes from dividing 5.00 by 4.00; a real yield can never exceed 100%.

On a pH-against-volume titration curve for a strong acid and a strong base, where is the equivalence point read off?

• At the midpoint of the steep vertical section
• At the very start of the curve
• Where the curve stays flattest near the very top
• At the first drop of alkali added

Answer: At the midpoint of the steep vertical section

The equivalence point lies at the centre of the steep near-vertical jump, where a tiny added volume causes a large pH change. For a strong acid and strong base this falls at pH 7.

Five repeat titres are 24.10, 24.15, 24.20, 24.65 and 24.15 cm³. Identify the correct treatment of the value 24.65 cm³.

• Exclude it from the mean and state in the report that it was excluded
• Include it in the mean, because every measurement taken must be used
• Delete it from the results table, so that only the concordant titres appear
• Repeat the whole titration and use only the second set of five results

Answer: Exclude it from the mean and state in the report that it was excluded

An anomaly is left out of the calculated mean but must still be reported, together with the reason for excluding it — usually that it falls well outside the concordant range. Deleting it from the table hides evidence, while including it drags the mean towards a value the other four trials contradict.

Identify what the length of a vertical error bar on a plotted point represents.

• The difference between that measurement and the accepted literature value
• The uncertainty in that measurement, drawn above and below the point
• The scatter of every point in the data set about the line of best fit
• The distance by which that point misses the drawn line of best fit

Answer: The uncertainty in that measurement, drawn above and below the point

An error bar shows the range within which the true value is expected to lie for that one point, so it comes from the apparatus and not from the other data. A line of best fit that passes through every error bar is consistent with the data; one that misses several is not.

Identify the correct way to present a set of repeat titres and their mean in a report.

• A bar chart with one bar per trial, so that the anomalous trial can be seen at a glance
• A table with the unit written after each individual value, in whatever precision that value was read
• A table with the unit and uncertainty in the heading and every value to the same decimal places
• A sentence listing the values in the order collected, followed by the calculated mean

Answer: A table with the unit and uncertainty in the heading and every value to the same decimal places

Units and uncertainties belong in the column heading, so they are stated once and apply to the whole column, and every raw value is quoted to the precision the apparatus delivered. A bar chart implies the trials are separate categories rather than repeats of one measurement.

Identify the reason a line of best fit is drawn through plotted points rather than joining them one to the next.

• A line of best fit must pass through the origin, whereas joined points need not
• A straight line is easier to read a gradient from than a series of short segments
• Joining the points would suggest the measurements were taken in a fixed order
• The scatter of the points comes from random error, which the line averages out

Answer: The scatter of the points comes from random error, which the line averages out

Each point carries an uncertainty, so the true relationship is a smooth trend that the scattered points estimate. Joining the points treats every random deviation as a real feature of the chemistry, which is why a dot-to-dot graph cannot support a conclusion.