Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations：練習題

Calculate the relative formula mass (\(M_{r}\)) of magnesium chloride, \(\text{MgCl}_{2}\).
(Relative atomic masses: \(\text{Mg} = 24\), \(\text{Cl} = 35.5\))

Calculate the relative formula mass (\(M_r\)) of calcium phosphate, \(\text{Ca}_3(\text{PO}_4)_2\).
(Relative atomic masses: \(\text{Ca} = 40\), \(\text{P} = 31\), \(\text{O} = 16\))

A mixture containing \(10\text{g}\) of hydrogen gas and \(32\text{g}\) of oxygen gas is ignited to form water:
\(2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}\)
Calculate the maximum mass of water that can be produced. (Relative atomic masses: \(\text{H} = 1\), \(\text{O} = 16\))

In an experiment, \(1.2\text{g}\) of magnesium (\(A_r = 24\)) reacted exactly with \(0.8\text{g}\) of oxygen gas (\(M_r = 32\)) to form magnesium oxide. Which of the following is the correctly balanced symbol equation for this reaction derived from these masses?

State the law of conservation of mass in terms of the atoms involved in a chemical reaction.

Calculate the percentage by mass of oxygen in calcium carbonate, \(\text{CaCO}_3\).
(Relative atomic masses: \(\text{Ca} = 40\), \(\text{C} = 12\), \(\text{O} = 16\))

A student reacts hydrochloric acid with calcium carbonate in an open beaker. Explain, in terms of the particle model, why the mass of the beaker and its contents appears to decrease.

A student carries out a thermal decomposition of magnesium carbonate (\(MgCO_3\)) in an open test tube. The balanced symbol equation for the reaction is:
\(MgCO_3(s) \rightarrow MgO(s) + CO_2(g)\)

(a) Explain, in terms of the law of conservation of mass and the particle model, why the mass of the solid remaining in the test tube appears to decrease during this reaction. (2 points)

(b) Calculate the relative formula mass (\(M_r\)) of magnesium carbonate.
(Relative atomic masses: \(Mg = 24, C = 12, O = 16\)) (1 point)

A student heated a crucible containing 2.40 g of magnesium ribbon. After the reaction was complete, the mass of the magnesium oxide produced was 4.00 g. The balanced equation for the reaction is:
\(2Mg(s) + O_2(g) \rightarrow 2MgO(s)\)

(a) Explain, in terms of the law of conservation of mass, why the mass of the solid in the crucible increased after heating. (2 points)

(b) Calculate the relative formula mass (\(M_r\)) of magnesium oxide, \(MgO\).
(Relative atomic masses, \(A_r\): \(Mg = 24\), \(O = 16\)). (1 point)

(c) Calculate the percentage by mass of magnesium in this sample of magnesium oxide. Show your working. (2 points)