Help with Chem question

Question

The combustion of methane has ΔH^o_comb = 890 kJ/mol. How many grams of molecular oxygen are equivalent to 1000kJ of heat released from this reaction?

Select one:

a.

35.9 g

b.

32.0 g

c.

72.0 g

d.

100.0 g

Answer 1

Look at the balanced equation for the combustion of methane:

CH₄ + 2O₂ ==> CO₂ + 2H₂O + 890 kJ heat ... balanced equation

Use the stoichiometry of the balanced equation along with dimensional analysis to find the grams of O₂ needed to generate 1000 kJ of heat.

1000 kJ x 2 mols O₂ / 890 kJ x 32 g O₂ / mol O₂ = 71.9 g O₂ = 72 g O₂ (Answer c)

Note: you should mention to who ever gave you this question that 1000 kJ has only 1 sig. fig. so the answer should actually be 70 g O₂ which has 1 sig. fig. (not 72.0 which has 3 sig. figs.).