Chemistry • Year 11 • Module 2 • Lesson 6

Concentration: Moles Per Litre

Build HSC Band 5–6 extended-response technique on multi-step concentration calculations, solution preparation procedure design, and critical evaluation of concentration claims in real contexts.

Master · Extended Response

1. Multi-step calculation — Great Barrier Reef water quality monitoring (Band 5–6)

8 marks   Band 5–6

Context. Scientists monitoring water quality on the Great Barrier Reef measure dissolved inorganic nitrogen (DIN). One indicator of excess nutrient run-off from agriculture is elevated ammonium ion (NH4+) concentration. A researcher collects a 500 mL water sample and determines it contains 0.0354 g of NH4+. Separately, water from a pristine reference site 50 km offshore contains 4.20 × 10−4 mol L−1 NH4+. (Molar masses: N = 14.01, H = 1.008)

Q1. Answer each part in full, showing all working and units.

(a) Calculate the molar mass of NH4+. (1 mark)

(b) Convert the 0.0354 g of NH4+ to moles, then calculate the concentration of the inshore sample in mol L−1. Show the mL→L conversion explicitly. (3 marks)

(c) Compare the inshore and offshore concentrations. Calculate the ratio (inshore ÷ offshore) and interpret what this means for the reef’s nutrient status relative to the pristine reference. (2 marks)

(d) What mass of NH4+ (in grams) would be found in one Olympic-sized swimming pool (2 500 000 L) at the pristine offshore concentration? State one assumption you make. (2 marks)

Stuck? (a) MM = 14.01 + 4(1.008) → (b) n = m ÷ MM, then c = n ÷ V(L) → (c) divide your answer by 4.20×10−4 → (d) n = c × V, then m = n × MM.

2. Experimental design — preparing and verifying a standard solution (Band 5–6)

7 marks   Band 5–6

Research question. A trainee laboratory technician at a Sydney university is asked to prepare exactly 250 mL of a 0.200 mol L−1 potassium nitrate (KNO3) solution to use as a standard reference in an ion chromatography calibration. They weigh out 5.05 g of KNO3, dissolve it in 250 mL of water in a beaker, and label it “0.200 mol L−1 KNO3”. A senior scientist tells them the preparation is incorrect.

Constraints: You have access to standard Year 11 / university laboratory equipment: analytical balance (0.001 g), 250 mL volumetric flask, beakers, wash bottle of distilled water, glass rod, funnels. (Molar masses: K = 39.10, N = 14.01, O = 16.00)

Q2. Evaluate the trainee’s preparation and design a correct procedure.

  • Calculate the mass of KNO3 required for exactly 250 mL of 0.200 mol L−1 and compare to the trainee’s mass of 5.05 g.
  • Identify the specific procedural error the trainee made and explain why it affects the concentration.
  • Write a correct step-by-step procedure for preparing 250 mL of 0.200 mol L−1 KNO3(aq) in at least five numbered steps, naming the equipment used at each step.
  • State one source of uncertainty in your procedure and describe how you would reduce it.
Stuck? Calculate: MM(KNO3) = 39.10+14.01+3(16.00). Required mass: n = c×V = 0.200×0.250 = 0.0500 mol; m = n×MM. Procedural error: adding water to 250 mL of solvent (not making up to the mark in a volumetric flask). Correct: dissolve in ~150 mL water, transfer to 250 mL volumetric flask, make up to the 250 mL mark.
Answers — Do not peek before attempting

Q1(a) — Molar mass of NH4+ (1 mark)

MM(NH4+) = 14.01 + 4(1.008) = 14.01 + 4.032 = 18.04 g mol−1. [1 mark; accept 18.0 g mol−1.]

Q1(b) — Moles and concentration of inshore sample (3 marks)

n = m ÷ MM = 0.0354 ÷ 18.04 = 1.962 × 10−3 mol (accept 1.96 × 10−3 mol) [1 mark].

V = 500 mL ÷ 1000 = 0.500 L [1 mark for explicit conversion].

c = n ÷ V = 1.962 × 10−3 ÷ 0.500 = 3.92 × 10−3 mol L−1 [1 mark; accept 3.93 or 3.9 × 10−3].

Q1(c) — Comparing concentrations (2 marks)

Ratio = 3.92 × 10−3 ÷ 4.20 × 10−4 = 9.3× (accept 9–10×) [1 mark]. The inshore sample has approximately 9× more NH4+ than the pristine reference, suggesting significant nutrient run-off from adjacent agricultural land has elevated nitrogen levels above the natural background, indicating potential reef stress [1 mark for interpretation].

Q1(d) — Mass in Olympic pool (2 marks)

n = c × V = 4.20 × 10−4 × 2 500 000 = 1050 mol.

m = n × MM = 1050 × 18.04 = 18 942 g ≈ 18.9 kg [1 mark correct calculation]. Assumption: the concentration is uniform throughout the pool volume (homogeneous solution) [1 mark for any valid assumption, e.g. no evaporation, temperature effect on density neglected].

Q2 — Sample Band 6 response (7 marks), annotated

Mass calculation: MM(KNO3) = 39.10 + 14.01 + 3(16.00) = 101.11 g mol−1 [1 mark]. n = c × V = 0.200 × 0.250 = 0.0500 mol. m = n × MM = 0.0500 × 101.11 = 5.056 g [1 mark]. The trainee’s mass of 5.05 g is essentially correct (rounds to 5.06 g to 3 sf); the error is procedural, not in the mass weighed.

Procedural error: The trainee dissolved KNO3 in 250 mL of water (i.e. added solute to 250 mL of solvent), rather than making the final solution up to a total volume of 250 mL. The total volume of the solution exceeds 250 mL because the dissolved solid increases the solution volume slightly; the actual concentration is therefore slightly less than 0.200 mol L−1 [1 mark for identifying error; 1 mark for explaining effect on concentration].

Correct procedure:

  1. Weigh 5.056 g of KNO3 on an analytical balance (0.001 g). Record the exact mass [equipment: analytical balance].
  2. Transfer the solid into a clean 250 mL beaker. Add approximately 100–150 mL of distilled water and stir with a glass rod until fully dissolved [equipment: beaker, glass rod, wash bottle].
  3. Allow the solution to cool to room temperature (dissolution may be endothermic), then transfer quantitatively to a 250 mL volumetric flask using a funnel. Rinse the beaker at least three times with small amounts of distilled water, adding all rinsings to the flask [equipment: 250 mL volumetric flask, funnel].
  4. Add distilled water carefully until the bottom of the meniscus sits exactly on the 250 mL calibration mark (viewed at eye level) [equipment: wash bottle, volumetric flask].
  5. Stopper the flask and invert at least 10 times to ensure thorough mixing. Label the flask with the solute, concentration, and date [equipment: stopper, label].

[1 mark for five clear, correctly sequenced steps; accept minor variations.] [1 mark for naming volumetric flask and describing making-up-to-mark step explicitly.]

Source of uncertainty and reduction: One source of uncertainty is evaporation from the beaker between weighing and transfer, which could reduce the mass of solid actually transferred. To reduce this, the solid should be transferred promptly and all rinsings used [1 mark for a specific, relevant uncertainty with a corresponding reduction strategy].

Marking criteria summary (7 marks): 1 = correct MM and required mass calculation; 1 = identifies and explains procedural error (dissolving in 250 mL instead of making up to 250 mL); 1 = explains effect on concentration (volume > 250 mL, c less than intended); 1 = correct five-step procedure in logical order; 1 = volumetric flask make-up-to-mark step explicitly described; 1 = rinsing of beaker step included; 1 = valid uncertainty and reduction strategy.

HSCScience • Chemistry • Year 11 • Module 2 • Lesson 6 • Worksheet 3 (Master)