What volume is occupied by 8.67 grams of methane gas at stp?

Molar mass methane ( CH₄) = 16.04 g/mol

number of moles:

mass CH₄ / molar mass

8.67 / 16.04 => 0.5405 moles

1 mole 22.4 L ( at STP )
0.5405 moles ??

0.5405 x 22.4 / 1 => 12.1072 L

hope this helps!

The mass has escaped into a gas which you can not see so the law stays true.