Topic 5: Formulae, equations and amounts of substance: Practice Questions

Which of the following is the correct definition for the molar mass of a substance?

A sample of a hydrated magnesium salt, \(MgSO_{4} \cdot xH_{2}O\), has a mass of 4.93 g. After heating to constant mass to remove all water of crystallisation, the remaining anhydrous magnesium sulfate has a mass of 2.41 g.
Calculate the value of \(x\) in the formula.
(Relative atomic masses: \(Mg = 24.3, S = 32.1, O = 16.0, H = 1.0\))

A mass of \(0.500\) g of a group 2 carbonate, \(MCO_{3}\), reacts completely with excess hydrochloric acid to produce \(135\) \(cm^{3}\) of carbon dioxide gas at \(298\) K and \(101\) kPa. Identify the metal \(M\).
(Gas constant \(R = 8.31\) J \(mol^{-1}\) \(K^{-1}\))

What volume, in \(dm^{3}\), is occupied by \(0.25\) mol of oxygen gas at room temperature and pressure (RTP)?
(Molar volume of a gas at RTP = \(24.0\) \(dm^{3}\) \(mol^{-1}\))

A sample of calcium phosphate, \(Ca_{3}(PO_{4})_{2}\), contains \(0.150\) mol of oxygen atoms.

What is the amount, in moles, of calcium phosphate in the sample?

State the numerical value of the Avogadro constant, including its units, and calculate the total number of atoms present in a sample of helium gas containing \(0.25\text{ mol}\).

A compound contains \(40.0\%\) carbon, \(6.7\%\) hydrogen, and \(53.3\%\) oxygen by mass. Calculate the empirical formula of this compound. (Relative atomic masses: \(C = 12.0, H = 1.0, O = 16.0\))

A student carries out a titration to find the concentration of a solution of sulfuric acid. They use a pipette with an uncertainty of \(\pm 0.06\text{ cm}^{3}\) to measure \(25.0\text{ cm}^{3}\) of the acid. Calculate the percentage uncertainty for this measurement and suggest one method to reduce the total apparatus uncertainty in a titration.

A sample of an unknown hydrated metal sulfate, \( MSO_{4} \cdot xH_{2}O \), has a mass of 5.14 g. After heating the sample to constant mass to remove all water of crystallisation, the mass of the anhydrous salt remaining is 2.82 g.

(a) Calculate the number of moles of water removed from the hydrated salt. (Relative atomic mass: \( H = 1.0, O = 16.0 \))
(b) If the value of \( x \) in the formula is known to be 7, calculate the relative atomic mass of the metal \( M \) and identify the element. (Relative atomic masses: \( S = 32.1, O = 16.0 \))