Quick answer: The mole is the chemist's counting unit: one mole contains 6.02 × 10²³ particles, the Avogadro constant. This Chemistry 9-10 lab on mole calculations gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
A mole is always the same number of particles, but is it always the same mass? In this lab you weigh out 0.10 mol of four kitchen substances, compare the measured masses with the values you calculate, and see how close an ordinary balance can come to an exact amount of substance.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-7, GCSE C3
Aim
To weigh 0.10 mol of four common substances and show that equal amounts of substance have very different masses.
Equipment
- Top-pan balance reading to 0.01 g, weighing boats, spatulas
- Table salt (sodium chloride), sugar (sucrose, C₁₂H₂₂O₁₁), baking soda (sodium hydrogencarbonate) and distilled water
- Calculator, goggles
Safety
Wear goggles. Do not taste any of the samples, even though they are kitchen chemicals. Wipe spills straight away and wash your hands at the end.
Method
- Use Ar values to calculate the molar mass of each substance, then the mass that equals 0.10 mol.
- Place a weighing boat on the balance and press tare (zero).
- Add the substance with a spatula until the display is as close as possible to the target mass and record the reading.
- Repeat for each substance, using a measuring cylinder and balance for the water.
Results
Sample results from Chiamaka’s group:
| Substance | M / g/mol | Target mass / g | Measured mass / g |
|---|---|---|---|
| Sodium chloride, NaCl | 58.5 | 5.85 | 5.87 |
| Sucrose, C₁₂H₂₂O₁₁ | 342 | 34.2 | 34.31 |
| Sodium hydrogencarbonate, NaHCO₃ | 84 | 8.40 | 8.38 |
| Water, H₂O | 18 | 1.80 | 1.82 |
Analysis questions
Question 1 Check the molar masses
Show how the molar mass of sucrose, C₁₂H₂₂O₁₁, and of sodium hydrogencarbonate are found (C 12, H 1, O 16, Na 23).Question 2 Target masses
Show that the target mass for 0.10 mol of NaCl is 5.85 g.Question 3 Actual amounts
Calculate the actual amount in moles for the measured mass of each substance.Question 4 Percentage difference
Calculate the percentage difference from 0.100 mol for water and for sodium hydrogencarbonate.Question 5 Number of particles
Calculate the number of formula units of sodium chloride in 0.100 mol.Question 6 Same number, different mass
All four samples contain about 6.02 × 10²² particles. Explain why their masses range from 1.8 g to 34 g.Question 7 Biggest error
Explain why water had the biggest percentage difference.
