how do i find the mass of each reactant needed to give the indicated amount of products?

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Chemistry is all a matter of

Chemistry is all a matter of proportions and ratios; the reactants (AgNO3 and CaCl2) react in a ratio of 2 to 1 and give products (AgCl and Ca(NO3)2 in the same proportion; thus if you only end up with 1.25 mol of AgCl, you will also end up with .75 mol of Ca(NO3)2 (to keep to original 2::1 ratio); and start out with 1.25 mol of AgNO3 and .75 mol of CaCl2. Thus, you multiply the molecular weight of AgNO3 by 1.25 and the molecular weight of CaCl2 by .75 to get the mass of the reactants. Now comes the hard part. Chem teachers come in two varieties: surfers and accountants. Surfers will let you estimate the atomic weight of Ag as 108 and Cl as 35.5, but accountants will insist that Ag is 107.868 and Cl is 35.453. For a surfer, the mass of the products is 287 gr AgCl (2 x 143.5,) but an accountant will expect an answer of 286.643 gr. (2 x 143.321) AgCl. The mass of the Ca(NO3)2 would be 40+ 2(14 + (3x16)) or around 164 grams. In your reaction, the AgCl mass would be 1.25 x 143.5 or about 179.4 grams and the Ca(NO3)2 would be .75 x 164 or around 123 grams. You can do similar calculations for the reactants after you figure out whether your teacher is a surfer or an accountant. When you do the calculations, the sum of the masses of the reactants should equal the sum of the masses of the products or about 302.4 grams.

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