Question

Five litres of gas at $STP$ have a mass of 12.5 $g$.What is the molecular mass of the gas?

• A. 12.5 $g/mol$
• B. 25.0 $g/mol$
• C. 47.5 $g/mol$
• D. 56.0 $g/mol$
• E. 125 $g/mol$

Answer 1

Answer: (D)

56.0 $g/mol$

No. of moles in five litres of gas at $STP$ $=$$5/22.4mol$

and moles$=$weight/molecular weight

so $5/22.4$$=$$12.5/Mw$

or $Mw$$=$$56g/mol$