AQA GCSE Chemistry (8462), Higher tier · Quantitative chemistry › Conservation of mass and chemical measurements
Practise Chemical measurements and uncertainty. 12 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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A student measured the temperature of a solution three times: 21.5 °C, 22.0 °C and 21.0 °C. Calculate the mean temperature.
21.5 °C
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Every measurement made with an instrument has some uncertainty.
(a) A balance shows the mass of a sample as 12.47 g. What is the resolution of the balance? Tick (✓) one box.[1]
0.01 g
0.1 g
1 g
12.47 g
(b) Give one reason why scientists repeat a measurement several times and calculate a mean.[1]
(c) A student measured the temperature of a solution three times: 21.5 °C, 22.0 °C and 21.0 °C. Calculate the mean temperature.[1]
(d) The uncertainty in the mean can be estimated as half the range of the readings. Calculate the uncertainty in the mean temperature.[1]
Show the answer and mark scheme
(a)Answer: 0.01 g
(b)
a single reading may be unusually high or low (anomalous) / repeating reduces the effect of random errors / repeating shows whether the results are repeatable
(c)Answer: 21.5 °C
21.5 (°C)
(d)Answer: ± 0.5 °C
range = 22.0 − 21.0 = 1.0 (°C), so the uncertainty is ± 0.5 (°C)
Question 2Medium5 marks
Whenever a measurement is made there is some uncertainty in the result.
(a) A student needs to measure 25.0 cm3 of sodium hydroxide solution as accurately as possible. Which piece of apparatus should the student use? Tick (✓) one box.[1]
Beaker
Conical flask
Measuring cylinder
Volumetric pipette
(b) The student measured a temperature using a thermometer with a resolution of 0.5 °C. What is meant by the resolution of a measuring instrument?[1]
(c) The student's balance was not set to zero. It read 0.12 g with nothing on it. The student used it to measure the masses of several samples. Name the type of error this causes and describe its effect on the measured masses.[2]
(d) Explain why finding the mass of a sample by difference (weighing a container before and after adding the sample) removes this error.[1]
Show the answer and mark scheme
(a)Answer: Volumetric pipette
(b)
the smallest change in the quantity that the instrument can measure / the size of the smallest scale division
(c)
systematic error / zero error
every mass reading is 0.12 g too high / all readings are wrong by the same amount
(d)
the extra 0.12 g is in both readings, so it cancels out when one reading is subtracted from the other
Question 3Hard5 marks
Two students, Priya and Sam, each measured the volume of gas produced when the same mass of calcium carbonate reacted with excess dilute hydrochloric acid. Priya used a measuring cylinder to collect the gas over water. Sam used a gas syringe. Each student repeated their method four times. Priya's mean was 62 ± 4 cm3. Sam's mean was 65.0 ± 0.5 cm3.
(a) Compare the precision of Priya's results with Sam's results. Use the data given.[2]
(b) Suggest one reason why a gas syringe gives more precise readings than collecting gas over water in a measuring cylinder.[1]
(c) The theoretical volume of gas expected, calculated from the mass of calcium carbonate used, was 65.8 cm3. State whether this value lies within the uncertainty of each student's mean.[2]
Show the answer and mark scheme
(a)
Sam's results are more precise (smaller uncertainty, ± 0.5 compared with ± 4)
Sam's repeat readings were closer together / less spread out than Priya's
(b)
allow: a gas syringe has a finer scale (better resolution) / less gas is lost or dissolves in the water when using a gas syringe / it is easier to read the scale accurately on a gas syringe
(c)
it lies within Priya's uncertainty (62 ± 4 covers 58 to 66 cm3)
it does not lie within Sam's uncertainty (65.0 ± 0.5 covers only 64.5 to 65.5 cm3)