Counting Particles by Mass: The Mole · Paper 1 — Free Chemistry SL Practice Questions

A student measures $4.00 \text{ g}$ of sodium hydroxide, NaOH, for a neutralisation reaction. The molar mass of NaOH is $40.00 \text{ g mol}^{-1}$ and Avogadro's constant is $6.02 \times 10^{23} \text{ mol}^{-1}$. How many formula units of NaOH are present in this sample?

A student reacts $0.24\ \text{g}$ of magnesium ribbon with excess hydrochloric acid. Given that the molar mass of magnesium is $24.0\ \text{g mol}^{-1}$ and Avogadro's constant is $6.02 \times 10^{23}\ \text{mol}^{-1}$, how many magnesium atoms are consumed in this reaction?

A student has 8.80 g of carbon dioxide gas ($M_r = 44.0$) in a sealed syringe. Exactly half of this gas is transferred to a reaction flask. How many molecules of $\text{CO}_2$ remain the syringe? ($N_A = 6.02 \times 10^{23}\ \text{mol}^{-1}$)

Magnesium reacts with excess hydrochloric acid according to the equation: $\text{Mg(s)} + 2\text{HCl(aq)} \rightarrow \text{MgCl}_2\text{(aq)} + \text{H}_2\text{(g)}$. A student uses $0.060\ \text{g}$ of magnesium ($A_r = 24.3$) and assumes 100% yield. Which expression gives the number of $\text{H}_2$ molecules collected?