Video Transcript
In a chemical reaction, five grams of A is found to completely react with seven grams of B. If the molar masses of A and B are 150 grams per mole and 140 grams per mole, what is the molar ratio of A to B?
In this problem, we have five grams of substance A, which has a molar mass of 150 grams per mole. This is reacted with substance B, which has a molar mass of 140 grams per mole. These two substances will react to form a product or products. The identity of the product or products that are formed are not important to know in order to solve this problem, as we are just looking for the molar ratio of A to B in this problem. The molar ratio of two substances is the ratio of the amount in moles of one substance to the amount in moles of another substance, which are both involved in a chemical reaction.
The molar ratio is represented by the stoichiometric coefficients in a balanced chemical equation, which are unknown in this problem. As we would like to determine the molar ratio, we’ll need the amount of each substance in moles. But right now we only have the mass of each substance. We could determine the amount of a substance in moles if we divide the mass by the molar mass. So the number of moles of A is equal to the mass of A, which is five grams, and then we divide by the molar mass of A, which is 150 grams per mole, which gives us 0.033 repeating moles of A, which we’ll round to 0.033.
Now we can calculate the moles of B. The mass of B is seven grams, and the molar mass of B is 140 grams per mole. Performing the calculation, we’ll find that we have 0.05 moles of B. Now that we’ve calculated the moles of A and B, we can calculate the molar ratio. Since we want the molar ratio of A to B, we want to divide the moles of A by the moles of B and not the other way around, as dividing the moles of B by the moles of A would be the ratio of B to A, which is not what we’re looking for.
From our calculations, we found that we had 0.033 moles of A and 0.05 moles of B. Performing the calculation, we get 0.6 repeating, or two-thirds. This means that every two moles of A react with three moles of B for a complete reaction. So we know the molar ratio of A to B is two to three.
