AgBr, a chemical used in photography, can be made with this reaction: 2AgNO3 + CaBr2 > 2 AgBr + CaN2O6 AgNO3 is very expensive, so making AgBr, you want to be sure that this reactant is all used up. If a chemist starts with 28.0g of AgNO3, how many grams of CaBr2 must be added to use up all of the AgNO3?

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2AgNO3 + CaBr2 --> 2AgBr + CaN2O6

We have 28 g of AgNO3.  We can see from the above balanced chemical equation that for every two moles of AgNO3 we need one mole of CaBr2.  So let's convert the mass of AgNO3 into moles.

28 g AgNO3 * (1 mole/169.87...

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2AgNO3 + CaBr2 --> 2AgBr + CaN2O6

We have 28 g of AgNO3.  We can see from the above balanced chemical equation that for every two moles of AgNO3 we need one mole of CaBr2.  So let's convert the mass of AgNO3 into moles.

28 g AgNO3 * (1 mole/169.87 g) = 0.165 moles AgNO3

We have stated that the molar ratio of AgNO3 to CaBr2 is 2:1 according to the balanced chemical equation.  So we divide the moles of AgNO3 by 2 to get the number of moles of CaBr2 needed to completely react with it.

0.165 moles AgNO3 * (1 mole CaBr2/2 moles AgNO3) = 0.0825 moles CaBr2

Now convert the moles of CaBr2 into grams.

0.0825 moles CaBr2 * (199.89 g/mole) = 16.5 g CaBr2.

Since silver is very expensive, we need to make sure that it is the limiting reagent in the reaction (in other words that it is not in excess and therefore wasted).  That means that we need to either have equimolar amounts of both starting reagents or have the CaBr2 be in excess.  In order for that to be true, we need to have at least 16.5 g of CaBr2 present in the reaction.

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