What is the gram formula mass of caso4.2h2o?

So your practically trying to finding the molar mass of CaSO₄.2H₂O and that is found by adding all the molar masses of the individual elements:

So Ca = 40 g/mol
      S  = 32 g/mol
      O = 16 /mol
      H = 1 g/mol

molar mass of CaSO₄.2H₂O = [(40) + (32) + (16 × 4) + (2 × 1 × 2) + (2 × 16)]
                                              = 172 g/mol

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