Question

If you weigh a sample of Magnesium metal to be 45g, how many moles do
you have?

Answer 1

`\boxed {\boxed {\sf 1.9 \ moles \ Mg}}`

Step-by-step explanation:

To convert from grams to moles, the molar mass is used. This tells us the grams in 1 mole of a specific substance. It can be found on the Periodic Table.

• Magnesium (Mg): 24.305 g/mol

Use the molar mass as a ratio.

`\frac { 24.305 \ g \ Mg}{1 \ mol \ Mg}`

Multiply by the given number of grams.

`45 \ g \ Mg *\frac { 24.305 \ g \ Mg}{1 \ mol \ Mg}`

Flip the fraction so the grams of magnesium can cancel.

`45 \ g \ Mg *\frac {1 \ mol \ Mg }{24.305 \ g \ Mg}`

`45 *\frac {1 \ mol \ Mg }{24.305 }`

`\frac {45 \ mol \ Mg }{24.305 }`

`1.85147089076 \ mol \ Mg`

The original measurement of grams has 2 significant figures, so our answer must have the same.

For the number of moles we calculated, that is the tenth place. The 5 in the hundredth place tells us to round the 8 to a 9.

`1.9 \ mol \ Mg`

45 grams of magnesium is about 1.9 moles.