Question

How many moles of Si are in 78.0 grams of silicon

Answer 1

The answer is 2.78 moles of Si

Answer 2

`\boxed {\boxed {\sf 2.78 \ mol \ Si}}}`

Step-by-step explanation:

To convert from grams to moles, we must use the molar mass. This number can be found on the Periodic Table.

• Si (Silicon): 28.085 g/mol

We can use number as a ratio.

`\frac { 28.085 \ g \ Si}{ 1 \ mol \ Si}`

Multiply by the given number of grams.

`78.0 \ g \ Si *\frac { 28.085 \ g \ Si}{ 1 \ mol \ Si}`

Flip the fraction so the grams of silicon cancel.

`78.0 \ g \ Si *\frac {1 \ mol \ Si }{ 28.085 \ g \ Si}`

`78.0 *\frac {1 \ mol \ Si }{ 28.085 }`

`\frac {78.0 \ mol \ Si }{ 28.085 }= 2.77728324729 \ mol \ Si`

The original number of grams (78.0) has 3 significant figures, so our answer must have the same. For the number of moles calculated, that is the hundredth place. The 7 in the thousandth place tells us to round the 7 to a 8.

`2.78 \ mol \ Si`

There are about 2.78 moles of silicon in 78.0 grams.