THE MOLE: Formulae and Chemical Equations: Combining volumes of gases

2.0 THE MOLE: Empirical and Molecular formula

2.14 Combining volumes of gases

Consider the general equation,

aX(g) + bY(g) = cZ(g)

For gases, the factors a, b, and c not only represent number of moles but also volumes. That is, we can as well write,

_v1X(g) + _v2Y(g) = _v3Z(g)

because the same number of moles of different gases occupy the same volume under the same conditions of temperature and pressure. Moreover, the volumes bear a simple (whole number) ratio to one another.

Gay Lussac^,s law: When gases combine, they do so in volumes that bear a simple ratio to one another and to products if gaseous.

Examples of reacting gases

1. 2H₂O(g) + O₂(g) = 2H₂O(g)
2. 2CO(g) + O₂(g) = 2CO₂(g)
3. 2NO(g) + O₂(g) = 2NO₂(g)
4. CH₄(g) + O₂(g) = H₂O(g) + CO₂(g)
5. N₂(g) + O₂(g) spark 2NO(g)
6. NH₃(g) + HCl(g) = NH₄Cl(s)
7. H₂(g) + Cl₂(g) = 2HCl(g)

Rewriting Equation 1 in terms of volume,
2volH₂(g) + 1volO₂(g) = 2volH₂O(g)

Questions 2.14

1. In the same manner shown above, rewrite Equation 2 in terms of volume.
2. Determine the volume of oxygen gas required to react completely with 12 dm³ of nitrogen (II) oxide (NO).
3. When a sample of methane gas (CH₄) was burnt completely in air, 8.0 dm³ of carbon (IV) oxide, measured at s.t.p was produced. Determine (a) the mass of methane burnt and (b) mass in g of steam produced (Molar gas volume at s.t.p. = 22.4 dm³; H = 1; O = 16).
4. A mixture of nitrogen (N₂) and oxygen (O₂) gases was sparked then allowed to cool to room temperature and pressure. The resulting mixture consisted of 100 cm³ of nitrogen (II) oxide and 60 cm³ of excess nitrogen gas. What were the initial volumes of oxygen and nitrogen?
5. A certain volume of ammonia gas combined directly with 80 cm³ of hydrogen chloride, measured at r.t.p. What was (a) the volume of ammonia used, and (b) mass of the product?
6. Explain why it would not be realistic to ask for the volume of product in Question 5?
7. A mixture of equal volumes of hydrogen and chlorine gases was reacted in a rigid container, and the product allowed to cool to the original temperature of the mixture. Explain why there was no overall change in pressure.

2.15 Project 2

Design an experiment to determine the water of crystallization, X, in a given hydrated salt, MSO₄.XH₂O. The teacher is requested to select and provide a suitable salt, with unfamiliar and concealed value of X, for this purpose.

In the design, explain clearly the procedure you would follow, the materials and apparatus you require and how you would use them, the measurements you would take, and how you would use them to obtain X. Hint: Water of crystallization is readily given off on heating.

Project work should preferably be done in a group.