13. Cu + S = CuS
What is the limiting reagent when 27 grams of S
reacts with 25 grams of Cu

Copper

Explanation:

To find limiting reactant, convert reactant mass values to moles & divide each by respective coefficient of balanced equation. The smallest resulting value is the limiting reactant.

        Cu             +              S                  =>         CuS

25g/64g·mol⁻¹          27g/32g·mol⁻¹  

= 0.391 mol Cu           = 0.844 mol S

*Note on working the problem => please see note below.

0.391/1                        0.844/1

= 0.391                        = 0.844

0.391 < 0.844 => Cu is limiting reactant*

*NOTE: The above is a simple way to ID the limiting reactant; however, one must work problem from mole values, NOT results of dividing by coefficients.  

i think it's letter a.