formula weight of phosphoric acid is 98.00g/mol, because:
mass of H = 1g
mass of O = 16g
mass of P = 31g
mass of H3PO4 = 3*1g + 31g + 4*16g = 98g/mol
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the density of 85% phosphoric acid is 1.685g/ml, so:
85% >>> we've 85g of phosphoric acid in 100g of solution (85g of acid; 100g-85g = 15g of water)
1.685g/ml >>> we've 1.685g of phosphoric acid solution in 1ml
when we dissolve 85g of phosphoric acid in 15g of water, we obtain 85% phosphoric acid which density is 1.685g/ml