Question

Calculate the number of moles in 32.0 g of methane, CH4, to 3 significant figures.

Answer 1

Answer:

2 moles

Explanation:

mass of ch4 is 16 g/mol

32g

32g/16 g/mol =

32/16 =

2

Answer 2

Answer:

$\boxed {\boxed {\sf 1.99 \ mol \ CH_4}}$

Explanation:

We are asked to calculate the number of moles in 32.0 grams of methane.

1. Molar Mass

We convert using the molar mass or mass of 1 mole of a substance. They are found on the Periodic Table because they are equal to the atomic masses, but the units are grams per mole instead of atomic mass units.

Look up the individual elements in methane: carbon and hydrogen.

• C: 12.011 g/mol
• H: 1.008 g/mol

Examine the formula of methane: CH₄. There is a subscript of 4 after H. There are 4 moles of hydrogen in 1 mole of methane. Multiply hydrogen's molar mass by 4 before adding carbon's.

• H₄: 1.008 * 4 =4.032 g/mol
• CH₄: 12.011 + 4.032 = 16.043 g/mol

2. Convert Grams to Moles

Now we convert grams to moles. We will use dimensional analysis, so we set up a conversion factor using the molar mass.

$\frac {16.043 \ g \ CH_4}{1 \ mol \ CH_4}$

We are converting 32.0 grams to moles, so we multiply by this value.

$32.0 \ g \ CH_4 *\frac {16.043 \ g \ CH_4}{1 \ mol \ CH_4}$

Flip the ratio so the units of grams of methane cancel.

$32.0 \ g \ CH_4 *\frac {1 \ mol \ CH_4}{16.043 \ g \ CH_4}$

$32.0 *\frac {1 \ mol \ CH_4}{16.043 }$

$\frac {32.0}{16.043} \ mol \ CH_4$

$1.994639407 \ mol \ CH_4$

3. Significant Figures

We must round our answer to 3 significant figures because the original measurement of grams has 3 sig fig.

For the number we found, that is the hundredth place. The 4 in the thousandth place tells us to leave the 9 in the hundredth place.

$1.99 \ mol \ CH_4$

32.0 grams of methane contains 1.99 moles of methane.