The method and the results are on the reacting masses lab. Do the analysis yourself first, then check the answers below.
Quick answer: Reacting masses come from a balanced equation. These answers go with the Chemistry 9-10 lab on reacting masses: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
The answers use the sample data. Your own values will differ a little, but the shape of the graph and the method for finding the concentration of the acid from the plateau must match.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: HS-PS1-7, GCSE C3
The graph is a straight line through the origin up to about 1.7 g, then a horizontal line at about 0.87 g. Mass lost is proportional to baking soda while the acid is in excess; after that, the acid is limiting.
Question 2 Find the turning point
Extend both lines until they cross. The sloping line through trials 1–4 and the plateau meet at about 1.7 g of NaHCO₃.
Question 3 Explain the plateau
In trials 5 and 6 all the acid has reacted. Extra NaHCO₃ is in excess and cannot release more CO₂, so the mass lost stays at 0.87 g.
Question 4 Moles of carbon dioxide
n = 0.87 ÷ 44 = 0.0198 mol (about 0.020 mol).
Question 5 Concentration of the vinegar
n(acid) = 0.0198 mol; V = 0.0250 dm³; c = 0.0198 ÷ 0.0250 = 0.79 mol/dm³.
Question 6 Percentage yield of CO₂
0.87 ÷ 0.88 × 100 = 98.9%.
Question 7 Sources of error
Too small: some CO₂ dissolves in the liquid and stays in the flask. Too large: water vapour leaves with the gas and adds to the apparent mass lost. Dissolved CO₂ and escaping vapour are the two main errors.
Question 8 Improve the lab
Take more trials between 1.4 g and 2.0 g, and repeat each trial to find a mean. More points near the turning point and repeats improve the estimate.
