Quantitative chemistry, Year 10: formula mass, uncertainty and moles

Relative formula mass (Mr)

提示Its symbol is a capital M with a small r.

为什么For water, H2O, it is 1 + 1 + 16 = 18. It has no unit because, like relative atomic mass, it is a comparison of masses and not a mass in grams.

58

提示The 2 outside the bracket applies to everything inside it.

为什么The formula contains one Mg, two O and two H: 24 + (2 × 16) + (2 × 1) = 58. Work out the bracket first, 16 + 1 = 17, then double it and add the magnesium.

28; 35

提示Find the part that is nitrogen, then express it as a fraction of the whole.

为什么There are two N atoms in the formula, one in NH4 and one in NO3, so nitrogen contributes 2 × 14. Dividing that by the relative formula mass and multiplying by 100 gives the percentage: 28 ÷ 80 × 100.

relative formula mass

提示You are comparing a part with the whole, so divide by the total for one unit of the compound.

为什么The top of the fraction is the Ar of the element multiplied by the number of its atoms in the formula. The bottom is the Mr of the whole compound. AQA expects you to be able to do this calculation when given the relative masses.

44, because the totals on each side of a balanced equation are equal

提示No atoms are made or lost, so whatever goes in must come out.

为什么In a balanced equation the relative formula masses of the reactants, in the amounts shown, add up to the same total as those of the products: 100 = 56 + 44. You can check it from the atoms: 12 + (2 × 16) = 44.

21.1; 0.6; 0.3

提示Add and share out for the first; subtract smallest from largest for the second; then split that in two.

为什么Mean = (21.0 + 21.4 + 20.8 + 21.2) ÷ 4. Range = 21.4 − 20.8. The readings lie within half the range either side of the mean, so the result can be written as the mean ± half the range.

uncertainty

提示It is the doubt about how close a reading is to the true value.

为什么No instrument and no person reading it is perfect, so a measured value is never known exactly. Scientists estimate how big the doubt is and report it with the result.

Group A's, because its readings have the smaller range

提示Compare how widely each set is scattered around the middle.

为什么Group A's readings span 0.4 cm³, so its mean is 25.0 ± 0.2 cm³. Group B's span 4.0 cm³, so its mean is 25.0 ± 2.0 cm³. The more closely the repeats agree, the more confident you can be in the mean.

The true value is likely to lie between 24.8 and 25.2 cm³

提示Do the sum once with a plus and once with a minus.

为什么The figure after the ± sign is the uncertainty. It gives the interval around the measured value within which the true value is expected to lie: from 25.0 − 0.2 up to 25.0 + 0.2.

One reading cannot show how much the results vary

提示A spread needs at least two points.

为什么The uncertainty is estimated from how far the repeat readings are scattered about their mean. With a single reading there is no scatter to look at, so there is no way to judge it.

The Avogadro constant

提示It is named after an Italian scientist, and its value is about 6 followed by 23 zeros.

为什么Its value is 6.02 × 10²³ per mole. One mole of any substance contains this number of the particles named.

2

提示First find the mass of one mole from the formula.

为什么The relative formula mass of CO2 is 12 + (2 × 16) = 44, so one mole has a mass of 44 g. Number of moles = mass ÷ Mr = 88 ÷ 44.

40; 10

提示Add up the three atoms, then remember that one mole has that many grams.

为什么Mr = 23 + 16 + 1. The mass of one mole in grams is numerically equal to the relative formula mass, so mass = moles × Mr = 0.25 × 40.

3.01 × 10²³

提示Half a mole holds half of what a whole mole does.

为什么Number of particles = moles × the Avogadro constant = 0.5 × 6.02 × 10²³. Only the 6.02 is halved; the power of ten stays the same.

Neither: they contain the same number

提示A mole is a counting unit, like a dozen.

为什么One mole of anything contains the Avogadro constant of the particles named: here atoms in one case and molecules in the other. The mole of carbon dioxide has a greater mass (44 g against 12 g) because each of its particles is heavier.