The method and the results are on the mole calculations lab. Do the analysis yourself first, then check the answers below.
Quick answer: The mole is the chemist's counting unit: one mole contains 6.02 × 10²³ particles, the Avogadro constant. These answers go with the Chemistry 9-10 lab on mole calculations: the graph, the calculations, the uncertainties and the conclusion explained, for all 7 analysis questions.
The answers use the sample masses. Your measured values will differ slightly. Check that each target mass equals 0.10 × M, then calculate the actual amount from the measured mass. Show units in every line and keep three significant figures where the data allow.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-7, GCSE C3
Question 1 Check the molar masses
Sucrose: 12 × 12 + 22 × 1 + 11 × 16 = 144 + 22 + 176 = 342 g/mol. Sodium hydrogencarbonate: 23 + 1 + 12 + 48 = 84 g/mol.342 g/mol and 84 g/mol.
Question 2 Target masses
m = n × M = 0.10 × 58.5 = 5.85 g.5.85 g
Question 3 Actual amounts
NaCl: 5.87 ÷ 58.5 = 0.1003 mol. Sucrose: 34.31 ÷ 342 = 0.1003 mol. NaHCO₃: 8.38 ÷ 84 = 0.0998 mol. Water: 1.82 ÷ 18 = 0.1011 mol.0.100 mol, 0.100 mol, 0.0998 mol and 0.101 mol
Question 4 Percentage difference
Water: (0.1011 − 0.100) ÷ 0.100 × 100 = +1.1 %. NaHCO₃: (0.09976 − 0.100) ÷ 0.100 × 100 = −0.24 %.+1.1 % and −0.24 %
Question 5 Number of particles
N = 0.100 × 6.02 × 10²³ = 6.02 × 10²².6.02 × 10²² formula units
Question 6 Same number, different mass
The particles have different masses, so the same number of particles has a different total mass. A sucrose molecule is much heavier than a water molecule, with M = 342 g/mol compared with 18 g/mol.Different particles have different masses.
Question 7 Biggest error
The target mass for water is only 1.80 g, so a balance error of 0.02 g is a larger fraction of the sample: 0.02 ÷ 1.80 = 1.1 %. For sucrose the same 0.01 g error is less than 0.1 % of 34.2 g.Small samples give larger percentage errors.
