Question

How many moles is 17.6 g NaOH?

Answer 1

Answer:

$\boxed {\boxed {\sf D. 0.440 \ mol \ NaOH}}$

Explanation:

To convert from moles to grams, the molar mass is used (mass of 1 mole). The values are the same as the atomic masses on the Periodic Table, but the units are grams per mole (g/mol) instead of atomic mass units.

1. Molar Mass

We are given the compound sodium hydroxide (NaOH) and we need to look up the molar masses of the individual elements.

• Na:  22.9897693 g/mol
• O: 15.999 g/mol
• H: 1.008 g/mol

The formula for the compound has no subscripts, so there is 1 mole of each element in 1 mole of the substance. We can simply add the molar masses.

• NaOH: 22.9897693 + 15.999 + 1.008 = 39.9967693 g/mol

This means there are 39.9967693 grams of sodium hydroxide in 1 mole.

2. Convert Grams to Moles

Use the molar mass we found as a ratio.

$\frac {39.9967693 \ g \ NaOH}{ 1 \ mol \ NaOH}$

Since we are converting 17.6 grams of NaOH to moles, we multiply by this value.

$17.6 \ g\ NaOH *\frac {39.9967693 \ g \ NaOH}{ 1 \ mol \ NaOH}$

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

$17.6 \ g\ NaOH *\frac {1 \ mol \ NaOH}{ 39.9967693 \ g \ NaOH}$

$17.6 *\frac {1 \ mol \ NaOH}{ 39.9967693}$

$\frac{17.6 }{ 39.9967693} \ mol \ NaOH$

$0.4400355406 \ mol \ NaOH$

3. Round

The original measurement of grams has 3 significant figures, so our answer must have the same. For the number we calculated, that is the thousandth place.

• 0.4400355406

The 0 in the ten thousandths place (in bold above) tells us to leave the 0 in the thousandth place.

$0.440 \ mol \ NaOH$

17.6 grams of sodium hydroxide are equal to 0.440 moles of sodium hydroxide.