Question

How many grams of calcium hydroxide will be needed to react completely with 10.0 g of phosphorus acid?
I assume you meant phosphorous acid, H3PO3.

2H3PO3 + 3Ca(OH)2 ==> Ca3(PO3)2 + 6H2O
10.0 g H3PO3/82 = 0.12 mols H3PO3. Now look at the balanced equation. Convert 0.12 mols H3PO3 to mols Ca(OH)2 this way.
0.12 mols H3PO3 x (2 mols Ca(OH)2/3 mols H3PO3) = 0.08 mols Ca(OH)2 needed. Then grams Ca(OH)2 = mols x molar mass = ?
Post your work if you get stuck.

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