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Different methods

Why are these two questions calculated in different ways, yet they are essentially the same question....

q1.png q2.png

The Zinc is the limiting reagent in the first question, and the hydroxide in the second.

Yet, even after using the molar ratios according to the formulas, they are worked out differently - namely the moles of HCl for the 2nd question is 0.02 x2 (which makes sense), but for the question, the AgNO3 (the reagent in excess is divided by two)----> I was expecting the moles of Zinc to be multiplied by 2 to get the no. of moles for AgNO3....Why is this??

Please help, I am very confused!! :confused::confused::confused::confused:
The zinc is not actually the limiting reagent in the first question, it is the AgNO3.

There is definitly more AgNO3 than zinc, but according to the reaction equation, you need TWO moles of AgNO3 to react with ONE mole of zinc.

Since you need two moles of AgNO3 for each mole of Zn, a ratio of 2:1, you can effectively half the amount of that reactant.

So it's like saying you have only (0.005/2 = 0.0025 moles of AgNO3 and 0.003 moles of zinc. That makes the AgNO3 the limiting reagent!

I hope that makes sense now!
(edited 10 years ago)

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