Question

Question #74ccc

Answer 1

The complete reaction will produce 0.0224 grams of hydrogen gas. A mass of 0.95 grams represents a percentage yield of 4240%, which is not possible. You cannot have a yield greater than 100%.

Explanation:

*When I did my calculations I left the full answers on the calculator and rounded everything at the end to three significant figures.

This is a single displacement redox reaction, where the aluminium displaces hydrogen, whilst being oxidised from 0 to the +3 state. Hydrogen is concurrently reduced from the 1+ to the 0 state.

To solve for the amount of hydrogen gas produced, we need to look at the balanced chemical equation and the stoichiometric ratios in which the reagents react.

`2Al_((s))+6HCl_((aq))rarr2AlCl_(3(aq))+3H_(2(g))`

Calculate the number of moles of each reagent using the formulae

`n=(m)/(MM)`

`C=n/V`

So we have

`n(Al)=1.84/26.98=0.0682" "mol`

`n(HCl)=CV=2.955*0.0075=0.0222" "mol`

The stoichiometric ratio in the chemical equation tells us that for every mole of Al, three moles of HCl react

`n(HCl):n(Al)=6:2=3:1`

Therefore, divide the number of moles of HCl we have by three to see how much Al is needed to react completely with it.

`(n(HCl))/3=0.0222/3=0.00739" "mol`

This tells us that we only need 0.00739 moles of Al to use up all the HCl. Ask yourself, do we have more or less than 0.00739 moles of Al?

We have 0.0682 moles of Al which is more than 0.00739. This means that HCl is the limiting reagent and Al is in excess. The limiting reagent dictates how much product we will get.

Next, look at the stoichiometric ratio of limiting reagent vs product

`n(HCl):n(H_2)=6:3=2:1`

This means that for every mole of HCl that reacts, 1/2 a mole of hydrogen gas will be produced

`n(H_2)=1/2n(HCl)=1/2*0.0222=0.0111" "mol`

Now we can determine the mass of hydrogen gas produced using the formula

`n=m/(MM)`

`MM(H_2)=2.02" "g/(mol)`

`m=n*MM=0.0111*2.02=0.0224" "g`

Finally, to find the percentage yield, use the following formula

`"% yield"="mass obtained"/"theoretical yield"*100%=0.95/0.0224*100%=4240`

This is saying that we have a 4240% yield, which is impossible due to the Law of Conservation of Mass! You cannot physically create matter out of nothing and so 100% is the maximum yield possible. You will not be able to obtain more than 0.0224 grams of hydrogen gas from this reaction!