Question

A major component of gasoline is octane (C8H18). When octane is burned in air, it chemically reacts with oxygen gas (O2) to produce carbon dioxide (CO2) and water (H2O). What mass of water is produced by the reaction of 7.72g of oxygen gas? Be sure your answer has the correct number of significant digits.

Answer 1

Answer:

The answer is 6.25g.

Explanation:

First create your balanced equation. This will give you the stoich ratios needed to answer the question:

2C8H18 + 25O2 → 16CO2 + 18H2O

Remember, we need to work in terms of NUMBERS, but the question gives us MASS. Therefore the next step is to convert the mass of O2 into moles of O2 by dividing by the molar mass:

7.72 g / 16 g/mol = 0.482 mol

Now we can use the stoich ratio from the equation to determine how many moles of H2O are produced:

x mol H2O / 0.482 mol O2 = 18 H2O / 25 O2

x = 0.347 mol H2O

The question wants the mass of water, so convert moles back into mass by multiplying by the molar mass of water:

0.347 mol x 18 g/mol = 6.25g

Answer 2

Answer:

3.13 grams of water is produced by the reaction of 7.72 grams of oxygen gas.

Explanation:

$2C_8H_{18} +25O_2\rightarrow 16CO_2+18H_2O$

Moles of oxygen gas = $\frac{7.72 g}{32 g/mol}=0.24125 mol$

According to reaction , 25 moles of oxygen gas gives 18 moles of water.

Then 0.24125 moles of oxygen will give:

$\frac{18}{25}\times 0.24125 mol=0.1737 mol$

Mass of 0.1737 moles of water :

$18 g/mol\times 0.1737 mol=3.1266 g\approx 3.13 g$

3.13 grams of water is produced by the reaction of 7.72 grams of oxygen gas.