Everything Science Grade 11

6.6 CHAPTER 6. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE

3. A calcium mineral consisted of 29.4% calcium, 23.5% sulphur and 47.1% oxygen by mass.
   Calculate the empiricalformula of the mineral.


4. A chlorinated hydrocarbon compound wasanalysed and found toconsist of 24.24% carbon,
   4.04% hydrogen and 71.72% chlorine. From another experiment the molecular mass was found
   to be 99 g· mol−^1. Deduce the empiricaland molecular formula.

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6.6 Molar Volumes ofGases

ESBBL

It is possible to calculatethe volume of one moleof gas at STP using whatwe now know about gases.

1. Write down the ideal gas equation

pV = nRT, therefore V =

nRT

p

2. Record the values thatyou know, making surethat they are in SI units
   You know that the gas isunder STP conditions. These are as follows:
   p = 101. 3 kPa = 101 300 Pa
   n = 1 mole
   R = 8. 31 J· K−^1 · mol−^1
   T = 273 K


3. Substitute these valuesinto the original equation.

V =

nRT

p

V =

1 mol× 8. 31 J· K−^1 · mol−^1 × 273 K

101 300 Pa

4. Calculate the volume of 1 mole of gas under these conditions The volume of 1 moleof gas at

Tip

The standard units used
for this equation areP
in Pa,V in m^3 andT in
K. Remember also that 1
000 cm^3 =1 dm^3 and 1
000 dm^3 =1 m^3.

STP is 22. 4 × 10 −^3 m^3 = 22. 4 dm^3.

Example 13: Ideal Gas

QUESTION

A sample of gas occupies a volume of 20 dm^3 , has a temperature of 280 K and has a pressure