Question

How many grams of water are produced when propane $\left( {{C_3}{H_8}} \right)$ burns with 12L of oxygen at STP?

Answer 1

We are already provided with the equation. We need to first write the balanced equation. Then calculate the moles of the reactants to further calculate the weight of water.

Formula used: ${\text{moles = }}\dfrac{{{\text{ given mass }}}}{{{\text{molar mass}}}}$

Complete step by step answer
We already know that here propane $\left( {{C_3}{H_8}} \right)$ burns with 12L of oxygen at STP.
The following equation is written as:
${C_3}{H_8} + 5{O_2} \to 3C{O_2} + 4{H_2}O$
Therefore, the problem doesn't state how much propane is burned, so we're going to assume that there is enough to use up all the oxygen.
To find out how much that is, we need to convert litres of ${O_2}$ to moles of ${O_2}$ using the fact that at STP, a mole of gas is equivalent to $22.4$ litres.
${{ = 12L \times }}\dfrac{{{{1mol }}}}{{{{22}}{{.4L}}}}$
${{ = 0}}{{.536 mol of }}{{{O}}_2}$
So, the molar ratio of oxygen to water is $\dfrac{5}{4}$ and the molecular weight of water is 18g.
Thus,
${\text{weight of }}{{{H}}_2}{{O = 0}}{{.536mol }}\left( {{O_2}} \right){{ \times }}\dfrac{{{{4 mol }}\left( {{H_2}O} \right){{ }}}}{{{{5 mol }}\left( {{O_2}} \right)}} \times 18.0{{g}}\left( {{H_2}O} \right)$
${{ = 7}}{{.72 g of }}{H_2}O$
So, the weight of ${H_2}O$ is $7.72{{g}}$ .

Additional Information
We should know that in order to calculate If two water samples have different volumes, they still share a common measurement: the density. Density is another measurement derived from SI basic units. The density of a material is defined as its mass per unit volume. We should know that in order to calculate the number of moles of any substance present in the sample, we simply divide the given weight of the substance by its molar mass. Where 'n' is the number of moles, 'm' is the given mass and 'M' is the molar mass. A mole is defined as the mass of the substance which consists of the equal quantity of basic units. Example atoms in 12 grams are the same as 12C. The basic units can be molecules, atoms or formula units based on the substance.
One mole of any substance is equal to the value of $6.023 \times {10^{23}}$ (Avagadro number). It can be used to measure the products obtained from the chemical reaction. The unit is denoted by mol.

Note
We already know that the molecular formula to find the molar mass; to obtain the number of moles, divide the mass of the compound by the molar mass of the compound expressed in grams.
All the values should be put carefully and the equations should be solved neatly.