Question

Mass- 83g

Volume before candle was burnt - 1000 mL

Volume after candle was burnt - 964.2 mL

Atmospheric pressure - 1.09 atm

Temperature - 296 Kelvins

1) Using the ideal gas law, calculate the number of moles of air originally present in the 250 mL flask

2) calculate the number of moles of air present in the flask after the candles burnt

3) calculate the number of moles of oxygen consumed based on the ideal gas law calculations

Answer 1

1) Volume before candle was burnt - 1000 mL

but in 250 ml flask only 250 ml air can occupied

so volume = 250 ml = 0.25 L

P V = n RT

1.09 x 0.25 = n x 0.0821 x 296

n = 0.0112

before burnt moles of O2 present = 0.0112 moles

2)

1000 (before voluem )---------------------> 964.2 (remaining after burn)

250 (flask ) ---------------------> ? (remaining in the flask)

remaining volume in 250 ml falsk after burnt = 964.2 x 250 / 1000

                                                                       = 241.05 ml

                                                                        = 0.241L

PV = n RT

1.09 x 0.241 = n x 0.0821 x 296

n = 0.0108

after burnt moles of O2 present = 0.0108 moles

3) volume of oxygen consumed = 250-241.05

                                                     = 8.95 ml

                                                     = 0.00895L

PV = nRT

1.09 x 0.00895 = n x 0.0821 x 296

n = 4 x 10^-4

4 x 10^-4 moles oxygen consumed .