Question

Calculate the no. of moles in 15g of CaCl2

Answer 1

`\boxed {\boxed {\sf 0.14 \ mol \ CaCl_2}}`

Step-by-step explanation:

We are asked to calculate the number of moles of 15 grams of calcium chloride (CaCl₂).

To convert from grams to moles, we use the molar mass, or the mass of 1 mole of a substance. Molar masses are found on the Periodic Table because they are equivalent to the atomic masses, but the units are grams per mole instead of atomic mass units.

Look up the individual elements in the compound: calcium and chloride.

• Ca: 40.08 g/mol
• Cl: 35.45 g/mol

Notice the chemical formula has a subscript of 2 after Cl or chlorine. There are 2 moles of chlorine in every 1 mole of calcium chloride. We must multiply chlorine's molar mass by 2 before adding calcium's molar mass.

• Cl₂: 35.45 * 2 = 70.9 g/mol
• CaCl₂= 40.08 + 70.9 = 110.98 g/mol

We will convert using dimensional analysis, so we must create a ratio using the molar mass.

`\frac {110.98 \ g \ CaCl_2}{ 1 \ mol \ CaCl_2}`

We are converting 15 grams of calcium chloride to moles, so we must multiply the ratio by this value.

`15 \ g \ CaCl_2 *\frac {110.98 \ g \ CaCl_2}{ 1 \ mol \ CaCl_2}`

Flip the ratio so the units of grams of calcium chloride cancel.

`15 \ g \ CaCl_2 *\frac { 1 \ mol \ CaCl_2}{110.98 \ g \ CaCl_2}`

`15 *\frac { 1 \ mol \ CaCl_2}{110.98}`

`\frac { 15}{110.98} \ mol \ CaCl_2`

`0.1351594882\ mol \ CaCl_2`

The original measurement of grams (15) has 2 significant figures, so our answer must have the same. For the number we calculated, that is the hundredth place. The 5 in the thousandth place tells us to round the 3 up to a 4.

`0.14 \ mol \ CaCl_2`

15 grams of calcium chloride is approximately 0.14 moles of calcium chloride.