Why mass is conserved, calculating relative formula mass, explaining apparent mass changes when a gas is involved, and handling uncertainty in measurements.
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1.State the law of conservation of mass.
No atoms are lost or made during a chemical reaction, so the total mass of the products equals the total mass of the reactants.
2.Why is mass always conserved?
The atoms are simply rearranged. The same atoms are present before and after, just joined together differently.
3.What is relative formula mass, Mr?
The sum of the relative atomic masses of all the atoms shown in the formula of a substance.
4.Calculate the Mr of water, H2O. H is 1, O is 16.
(2 x 1) + 16 = 18
5.Calculate the Mr of carbon dioxide, CO2. C is 12, O is 16.
12 + (2 x 16) = 44
6.Calculate the Mr of calcium carbonate, CaCO3. Ca is 40, C is 12, O is 16.
40 + 12 + (3 x 16) = 100
7.Calculate the Mr of magnesium hydroxide, Mg(OH)2. Mg is 24, O is 16, H is 1.
24 + 2 x (16 + 1) = 24 + 34 = 58
8.What is relative atomic mass, Ar?
The average mass of the atoms of an element compared with one twelfth the mass of a carbon-12 atom, taking isotope abundance into account.
9.In a balanced equation, how do the total Mr values compare?
The total relative formula mass of the reactants equals that of the products, once the balancing numbers are included.
10.Why might the mass appear to decrease during a reaction in an open container?
One of the products is a gas, and it escapes into the air so it is no longer being weighed.
11.Give an example of a reaction where mass appears to decrease.
Heating a metal carbonate, which gives off carbon dioxide gas.
12.Why might the mass appear to increase during a reaction in an open container?
One of the reactants is a gas from the air, such as oxygen, and it becomes part of the solid product.
13.Give an example of a reaction where mass appears to increase.
Burning magnesium in air, where oxygen from the air combines with the magnesium.
14.How could you show that mass really is conserved when a gas is produced?
Carry out the reaction in a sealed container so nothing can enter or escape, and weigh it before and after.
15.What is a mean, and why is it used?
The sum of the readings divided by how many there are. It reduces the effect of random error in individual measurements.
16.What is the range of a set of results?
The difference between the highest and lowest value.
17.How is uncertainty estimated from a set of repeats?
It is taken as half the range of the repeated readings.
18.Readings are 24, 26 and 25 cm3. Calculate the mean and the uncertainty.
Mean = 75 divided by 3 = 25 cm3. Range = 26 minus 24 = 2, so uncertainty = 1 cm3. The result is 25 plus or minus 1 cm3.
19.What is an anomalous result?
A reading that does not fit the pattern of the others, usually caused by a mistake in the measurement.
20.What should be done with an anomalous result?
It should be ignored when calculating the mean, and if possible the measurement should be repeated.
21.What is meant by a precise measurement?
Repeated readings that are all close together, whether or not they are close to the true value.
22.What is meant by an accurate measurement?
A reading that is close to the true value.
23.Why do all measurements have some uncertainty?
Every instrument has a limit to how finely it can be read, and there is always some variation between repeats.
24.What does the number of significant figures in an answer depend on?
The precision of the data used. An answer should not be given to more significant figures than the least precise measurement.
25.Balance this and check the masses: Mg + O2 gives MgO
2Mg + O2 gives 2MgO. Reactants: (2 x 24) + 32 = 80. Products: 2 x 40 = 80. Mass is conserved.
26.If 10 g of a reactant produces 6 g of solid product in an open flask, what happened to the other 4 g?
It was released as a gas and escaped from the flask.
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