This reaction is used to produce a small amount of magnesium oxide for a demonstration in a school laboratory. A student heats magnesium ribbon in a crucibl...
Question 1 Report
This reaction is used to produce a small amount of magnesium oxide for a demonstration in a school laboratory. A student heats magnesium ribbon in a crucible with its lid slightly raised. The reaction is 2Mg + O2 → 2MgO. Table 1 shows the masses before and after heating. Relative atomic masses are Mg = 24 and O = 16.
| measurement | mass / g |
|---|---|
| empty crucible and lid | 24.62 |
| crucible, lid and magnesium before heating | 25.82 |
| crucible, lid and product after heating | 26.42 |
The student repeats the heating until the mass stays the same. This helps to ensure that the reaction has finished.
(a) Use Table 1 to calculate the mass of magnesium used. [1]
(b) What mass of magnesium oxide was made? [1]
(c) Explain why repeated heating to constant mass improves the result. [1]
(d) Use the formula MgO to calculate the mass of 0.050 mol of magnesium oxide. [2]
A jeweller melts a tiny amount of pure gold to make a calibration bead for a balance. The bead has a mass of 3.94 g. Gold is an element, so its formula is Au. The relative atomic mass of gold is 197. The jeweller records the amount of substance as well as the mass because calibration beads of different metals contain different numbers of atoms for the same mass. No oil or polishing compound remains on the bead.
(a) Name the element represented by Au. [1]
(b) Use the relative atomic mass to calculate the amount, in mol, of gold in the bead. [2]
(c) Describe what is meant by one mole of gold atoms. [1]
(d) What mass of gold would contain 0.0500 mol of atoms? [1]
Answer Details
Magnesium oxide experiment
(a) Subtract the mass of the empty crucible and lid from the mass before heating:
\[25.82\text{ g}-24.62\text{ g}=1.20\text{ g}\]
The mass of magnesium used is 1.20 g. [1]
(b) Subtract the mass of the empty crucible and lid from the final mass:
\[26.42\text{ g}-24.62\text{ g}=1.80\text{ g}\]
The mass of magnesium oxide made is 1.80 g. [1]
(c) Heating until constant mass shows that all the magnesium has reacted, so no further oxygen is being gained. This makes the final product mass more reliable. [1]
(d) First find the relative formula mass:
\[M_r(\mathrm{MgO})=24+16=40\]
\[\text{mass}=0.050\text{ mol}\times40\text{ g mol}^{-1}=2.0\text{ g}\]
The mass of 0.050 mol of magnesium oxide is 2.0 g. [2]
Gold calibration bead
(a) \(\mathrm{Au}\) represents gold. [1]
(b) Gold is an element, so its molar mass is 197 g mol-1. Use \(\text{amount}=\frac{\text{mass}}{M_r}\):
\[\text{amount of Au}=\frac{3.94\text{ g}}{197\text{ g mol}^{-1}}=0.0200\text{ mol}\]
The bead contains 0.0200 mol of gold. [2]
(c) One mole of gold atoms contains \(6.02\times10^{23}\) gold atoms, the Avogadro constant number of atoms. [1]
(d) Use \(\text{mass}=\text{amount}\times M_r\):
\[\text{mass}=0.0500\text{ mol}\times197\text{ g mol}^{-1}=9.85\text{ g}\]
The mass is 9.85 g. [1]
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