The method and the results are on the atmospheric pressure lab. Do the analysis yourself first, then check the answers below.
Quick answer: Air is a mixture of gases, mainly 78 % nitrogen and 21 % oxygen, and it has mass: about 1.2 kg per cubic metre. These answers go with the grade 7 science lab on atmospheric pressure: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
Here are the atmospheric pressure lab answers for grade 7 science: graph, calculations, uncertainties and conclusion. First do the lab yourself, then compare, and go back to the lesson if needed.
The sample results give about 0.60 g of air per stroke, which matches 0.50 L at 1.2 g per litre. Use these model answers to check your calculations and compare the gradient of your own graph.
Atmospheric pressure lab answers: the analysis explained
Question 1 Total gain
416.3 – 392.4 = 23.9 g.
Question 2 Mass per stroke
23.9 ÷ 40 = 0.5975, so about 0.60 g per stroke.
Question 3 The graph
The points lie close to a straight line, so the mass added is proportional to the number of strokes. The gradient is 23.9 ÷ 40 = 0.60 g per stroke.
Question 4 Volume of one stroke
V = m ÷ ρ = 0.60 ÷ 1.2 = 0.50 L.
Question 5 Prediction
Mass = 392.4 + 60 × 0.60 = 392.4 + 36.0 = 428.4 g (about 428 g).
Question 6 Sources of error
Some air escapes while the pump is disconnected, and the pump may not push out its full volume at each stroke. Leaks and incomplete strokes.
Question 7 Conclusion
The mass of the ball rose steadily as air was added, so air has mass.
Question 8 Better method
Fit a one-way valve so no air escapes when the pump is removed, then repeat the whole run three times and average the gradients. Stop leaks and repeat.
