Skills in the study of chemistry practice questions.

A burette is graduated in 0.1 cm³ divisions. To what precision should each burette reading be recorded?

• 0.05 cm³
• 0.10 cm³
• 0.50 cm³
• 0.25 cm³

Answer: 0.05 cm³

A reading is estimated to half the smallest division, so a 0.1 cm³ burette is read to 0.05 cm³. Recording only to 0.1 cm³ throws away a significant figure the instrument can actually resolve.

Which apparatus measures the volume of gas evolved in a reaction most precisely?

• A gas syringe
• A conical flask
• A measuring cylinder over water
• A volumetric pipette

Answer: A gas syringe

A gas syringe reads gas volume directly with fine graduations and no gas dissolving or escaping. Collecting over water loses any soluble gas and is read less precisely; a pipette and flask do not measure evolved gas at all.

State one advantage of using a pH data-logging probe rather than universal indicator paper.

• It gives continuous quantitative readings over time
• It changes colour more clearly than indicator paper ever could
• It removes the need to calibrate before use
• It measures amount of substance directly in moles

Answer: It gives continuous quantitative readings over time

A probe logs pH numerically and automatically at set intervals, capturing rapid changes a person would miss. It still needs calibrating against buffers, and it measures pH, not amount of substance, so the other options are false.

In a spreadsheet, what is the best way to obtain the gradient of a best-fit straight line through experimental data?

• Apply the SLOPE function to the y- and x-columns
• Divide the final y-value by the corresponding final x-value
• Take the arithmetic mean of all of the y-values
• Add together all the values in the x-column

Answer: Apply the SLOPE function to the y- and x-columns

The SLOPE function performs a least-squares fit over all data points, using every measurement to find the gradient. Using a single point ignores the other data and any intercept, and averaging or summing a column does not give a gradient at all.

Express the concentration 0.00042 mol dm⁻³ in standard form.

• 4.2 × 10⁻⁴ mol dm⁻³
• 4.2 × 10⁻³ mol dm⁻³
• 42 × 10⁻⁵ mol dm⁻³
• 4.2 × 10⁴ mol dm⁻³

Answer: 4.2 × 10⁻⁴ mol dm⁻³

Moving the decimal point four places to the right gives 4.2, so the number is 4.2 × 10⁻⁴. Standard form also requires the coefficient to be between 1 and 10, which rules out the 42 × 10⁻⁵ option even though it has the same value.

A student changes the temperature and measures the time for a cross to disappear under a flask. Identify the independent variable.

• The temperature
• The time for the cross to disappear
• The concentration of the acid used
• The volume of the reaction mixture

Answer: The temperature

The independent variable is the one deliberately changed by the experimenter, here temperature. The time measured is the dependent variable, and concentration and volume are held constant as controlled variables.