Video: Calculating the Masses of Products from the Masses of Reactants Where One Reactant Is in Excess

According to the following equation, how many grams of HCl are obtained from 71 g of Cl₂ and 6 g of H₂? Cl₂ + H₂ ⟶ 2HCl. [A] 36.5 grams of HCl are obtained with two grams of hydrogen gas in excess. [B] 36.5 grams of HCl are obtained with six grams of hydrogen gas in excess. [C] 77 grams of HCl are obtained without reactants left over. [D] 71 grams of HCl are obtained with four grams of hydrogen gas in excess. [E] 73 grams of HCl are obtained with four grams of hydrogen gas in excess.

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Video Transcript

According to the following equation, how many grams of HCl are obtained from 71 grams of Cl₂ and six grams of H₂? Cl₂ plus H₂ react to form 2HCl. A) 36.5 grams of HCl are obtained with two grams of hydrogen gas in excess. B) 36.5 grams of HCl are obtained with six grams of hydrogen gas in excess. C) 77 grams of HCl are obtained without reactants left over. D) 71 grams of HCl are obtained with four grams of hydrogen gas in excess. Or E) 73 grams of HCl are obtained with four grams of hydrogen gas in excess.

We’re dealing with the reaction of chlorine gas and hydrogen, forming hydrogen chloride. In order to work out the mass of hydrogen chloride we produce, we need to work out which component out of chlorine and hydrogen is in excess. And then work out the number of moles of HCl from that. The question is using round numbers. So we can use the atomic masses for chlorine and hydrogen in their round number form. The atomic mass of chlorine is 35.5 unified atomic mass units. So the molar mass of the chlorine molecule is equal to two times 35.5 grams per mole. So the molar mass of chlorine is 71 grams per mole. We can work out the amount of chlorine gas in 71 grams of chlorine gas by multiplying 71 by one over 71. You can also write it as 71 grams divided by 71 grams per mole. So we have one mole of chlorine.

The molar mass of dihydrogen is equal to two times the atomic mass of hydrogen in grams per mole. So it’s equal to two times one grams per mole. We can calculate the amount of hydrogen in six grams of hydrogen by taking six grams and multiplying it by one mole per two grams. You can get the same value by dividing six grams by two grams per mole. Whichever way you do it, you should get three moles of hydrogen. Molecules of chlorine and hydrogen react in a ratio of one to one. So we consume one mole of chlorine, one mole of hydrogen to produce two moles of hydrogen chloride. This means once our reaction is complete, we’re left with two moles of hydrogen and two moles of hydrogen chloride. The molar mass of HCl is one plus 35.5 grams per mole, giving us 36.5 grams per mole. We can work out the mass of our two moles of hydrogen chloride by multiplying the molar mass, 36.5 grams per mole, by two moles. This gives us 73 grams. The only statement that has 73 grams of HCl in it is E.

But let’s just check the amount of hydrogen we get just in case. We can confirm the mass of hydrogen we have in excess by the end by multiplying the molar mass of hydrogen, two grams per mole, by the number of moles we have, two. This gives us four grams of hydrogen in excess. So according to the equation, Cl₂ plus H₂ react to form 2HCl. We will get 73 grams of HCl and four grams of hydrogen gas in excess when we react together 71 grams of Cl₂ and six grams of H₂.

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