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how do you calculate theoretical yield of something? I don't know where to find the formula

You go to the balanced chemical equation, and look at the mole ratio between the reactants and products.

The theoretical yield is the value calculated from stoichiometry of a reaction. For example, in the reaction
2H2 + O2 ==> 2H2O, we might ask what the theoretical yield of H2O is if we start with 2 grams H2.
2 grams H2 = g/molar mass = 2/2 = 1 mol.

2 mols H2 will produce 2 mols H2O and we know that by looking at the equation. Therefore, 1 mol H2 will form 1 mol H2O. Then, 1 mol (2 grams) is the theoretical yield for water if we start with 2 grams H2. Usually, when we talk about theoretical yield, we also talk about percent yield.
%yield = [actual yield)/theoretical yield] x 100.

In the above problem, suppose we said that we performed the experiment and we obtained 1.8 grams of water. Then the percent yield would be
%yield water = (1.8/2.0)x 100 = 90%.

I hope this helps.

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