Answer:
[tex]\boxed {\boxed {\sf 2.78 \ mol \ Si}}}[/tex]
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.
[tex]\frac { 28.085 \ g \ Si}{ 1 \ mol \ Si}[/tex]
Multiply by the given number of grams.
[tex]78.0 \ g \ Si *\frac { 28.085 \ g \ Si}{ 1 \ mol \ Si}[/tex]
Flip the fraction so the grams of silicon cancel.
[tex]78.0 \ g \ Si *\frac {1 \ mol \ Si }{ 28.085 \ g \ Si}[/tex]
[tex]78.0 *\frac {1 \ mol \ Si }{ 28.085 }[/tex]
[tex]\frac {78.0 \ mol \ Si }{ 28.085 }= 2.77728324729 \ mol \ Si[/tex]
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.
[tex]2.78 \ mol \ Si[/tex]
There are about 2.78 moles of silicon in 78.0 grams.
