What mass in grams would 5.7L of hydrogen gas occupy at STP?

 The correct answer is:  " 0.54 g " .

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Explanation:

Note that "hydrogen gas" is:  

H₂ (g)  ;   that is:  a "diatomic element" (diatomic gas) ;

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The molecular weight of "H" is:  1.00794 g ;

   (From the Periodic Table of Elements).

So, the molecular weight of:  H₂ (g)  is:

    " 1.00794 g * 2  = 2.01588 g ; {use calculator) ;

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Note the conversion for a gas at STP:

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  1 mol of a gas = 22.4 L gas;

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i.e. " 1 mol / 22.4 L " ;

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So:     " 5.7 L H₂ (g)  

The "L" ("literes" cancel out to "1" ;  since "L/L = 1 ;

The "mol" (moles) cancel out to "1" ; since "mol/mol = 1 ;

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and we are left with:

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 [5.7 * 2.104588 g ] / 22.4  =  ?  g ;

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→ [ 11.9961516  g ] / 22.4 =

          0.53554248214  g ;l

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We round this value to:  " 0.54 g " ;

 → since "5.7 L " has 2 (two) significant figures;  

     22.4 is an exact number conversion;

     and "5.7 L" has fewer significant figures than:

    " 2.104588 " ; or:  " 1.00794 " .

  → as such: We round to "2 (two) significant figures."

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Hope this is helpful.  Wishing you the best in your academic endeavors!

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