Which is the formula mass of Na sub 2 SO sub 4 ?

Add the atomic masses of all atoms in Na₂SO₄: 2 sodium atoms contribute 2 × 22.99 = 45.98 amu, sulfur contributes 32.06 amu, and 4 oxygen atoms contribute 4 × 16.00 = 64.00 amu. The total is 45.98 + 32.06 + 64.00 = 142.04 amu, which rounds to 142.05 amu. Therefore, the formula mass is choice A, 142.05 amu.

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