1.
State what is meant by n = m/M.
[1 mark]
- moles = mass / molar mass
JD SCIENCE
Laboratory chemical calculations
Indicative marking points. Award marks for equivalent scientific or mathematical wording. Do not issue this sheet with the student worksheet.
2.
State what is meant by c = n/V.
[1 mark]
- concentration in mol dm⁻³
3.
State what is meant by a dilution.
[1 mark]
- c1V1 = c2V2
4.
State what is meant by a titration calculation.
[1 mark]
- moles of known, ratio, moles of unknown, then c or m
5.
State what is meant by percentage yield.
[1 mark]
- actual / theoretical × 100
6.
State what is meant by atom economy.
[1 mark]
- Mr desired / Mr all products × 100
7.
State what is meant by a standard solution.
[1 mark]
- known accurate concentration
8.
State what is meant by uncertainty.
[1 mark]
- combine pipette, flask and burette errors
9.
State what is meant by a calibration graph.
[1 mark]
- absorbance vs concentration
10.
State what is meant by unit prefixes.
[1 mark]
- m, µ, n, k, M
11.
Noah is asked about n = m/M in a NCFE Laboratory Sciences paper.
(a) [2 marks]
- use consistent grams
(b) [3 marks]
- first step in most calculations
12.
Noah is asked about c = n/V in a NCFE Laboratory Sciences paper.
(a) [2 marks]
- V must be in dm³
(b) [3 marks]
- 250 cm³ = 0.250 dm³
13.
Noah is asked about a dilution in a NCFE Laboratory Sciences paper.
(a) [2 marks]
- use a pipette and flask
(b) [3 marks]
- mix thoroughly
14.
Noah is asked about a titration calculation in a NCFE Laboratory Sciences paper.
(a) [2 marks]
- concordant titres
(b) [3 marks]
- mean of closest values
15.
Noah is asked about percentage yield in a NCFE Laboratory Sciences paper.
(a) [2 marks]
- losses on transfer
(b) [3 marks]
- never over 100% if calculated correctly
16.
Noah is asked about atom economy in a NCFE Laboratory Sciences paper.
(a) [2 marks]
- green chemistry
(b) [3 marks]
- independent of yield
17.
Dilute 10.0 cm³ of 1.00 mol dm⁻³ to 250 cm³. New c?
[3 marks]
- Final answer: 0.0400 mol dm⁻³
18.
25.0 cm³ of 0.100 mol dm⁻³ HCl vs NaOH. Moles HCl?
[3 marks]
- Final answer: 0.00250 mol
19.
If that NaOH titre is 20.0 cm³, c(NaOH)?
[3 marks]
- Final answer: 0.125 mol dm⁻³
20.
Mass of 0.0500 mol of H₂SO₄ (M=98)
[3 marks]
- Final answer: 4.90 g
21.
Convert 250 µg to grams
[3 marks]
- Final answer: 2.50 × 10⁻⁴ g
22.
A graph reads 0.35 absorbance = 7.0 mg dm⁻³. What is 0.70 if linear through 0?
[3 marks]
- Final answer: 14 mg dm⁻³
23.
Noah carries out a T-Level occupational task on laboratory chemical calculations. This is a typical NCFE required-practical style question.
(a) [2 marks]
- Independent: the factor being investigated
- Dependent: the measured result or quality check
(b) [2 marks]
- follow the SOP and keep other variables constant
- so the test is fair / only one variable is changed
(c) [2 marks]
- not recording the batch/lot number or operator ID
24.
Noah carries out writing or following an SOP for laboratory chemical calculations. This is a typical NCFE required-practical style question.
(a) [2 marks]
- Independent: a step in the method
- Dependent: whether the acceptance criteria are met
(b) [2 marks]
- same calibrated instrument
- so the test is fair / only one variable is changed
(c) [2 marks]
- skipping a verification check
25.
Noah carries out a COSHH / risk-assessment review for laboratory chemical calculations. This is a typical NCFE required-practical style question.
(a) [2 marks]
- Independent: hazard
- Dependent: control measure effectiveness
(b) [2 marks]
- same PPE and waste route
- so the test is fair / only one variable is changed
(c) [2 marks]
- using expired reagents or the wrong waste stream
26.
Noah carries out data review in a LIMS or quality log for laboratory chemical calculations. This is a typical NCFE required-practical style question.
(a) [2 marks]
- Independent: sample ID
- Dependent: result vs specification
(b) [2 marks]
- same calculation method
- so the test is fair / only one variable is changed
(c) [2 marks]
- transcription error between the instrument and the LIMS
27.
Compare n = m/M with a titration calculation.
[4 marks]
- moles = mass / molar mass
- moles of known, ratio, moles of unknown, then c or m
- state one workplace implication
28.
Compare c = n/V with percentage yield.
[4 marks]
- concentration in mol dm⁻³
- actual / theoretical × 100
- state one workplace implication
29.
Compare a dilution with atom economy.
[4 marks]
- c1V1 = c2V2
- Mr desired / Mr all products × 100
- state one workplace implication
30.
Compare a titration calculation with a standard solution.
[4 marks]
- moles of known, ratio, moles of unknown, then c or m
- known accurate concentration
- state one workplace implication
31.
Apply your knowledge of a standard solution to this situation: primary standard if possible
[3 marks]
- primary standard if possible
32.
Apply your knowledge of uncertainty to this situation: percentage then add
[3 marks]
- percentage then add