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# AS Chemistry Practice Question Help watch

1. Hi,

I've tried to do these practice chemistry questions but I am really confused particularly with the 1st one as I don't really know what I'm doing! Would someone kindly point me in the right direction? Also with atom economy how do you know if a product is useful or wasted?

Thanks
2. Here is the sheet, I forgot to attach it
Attached Images

3. For the first question:

Calculate moles of calcium phosphate which has reacted
From the balanced chemical equation you can see that for every mole of calcium phosphate that reacts , twice as many moles of phosphoric acid will form
Therefore moles of acid produced will just be double moles of calcium phosphate reacted
This is called using the molar ratio of the reaction
Then use the moles=concentration × volume formula which is used when working with solutions
Rearrange it and use the information u have found and are given to calculate the concentration of phosphoric acid

For the atom economy question the question has hinted that calcium nitrate is the desired product so phosphoric is considered waste
Caclaute Mr of desired product and of all products in total then stick into the formal and multiply by 100

I have calculated answers let me now if u want them as I can't be 100 percent sure they're right!

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4. Have u managed to do part c?

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5. Thank you! That is such a great help as I was on the right track but had got myself in a muddle. I would be really grateful for the answers so that I can see if I get the right answer the 2nd time around.

I've done part c but I'm not sure it it's right. I first found out the percentage of each element in the compound, used this to find the moles and then found the simplest ratio.
6. Here's my working
Part a)
Moles of calcium phosphate = 7.26 ÷ 310.3= 0.023 moles
Therfore oles of phosphoric acid will be 0.023×2 = 0.047 moles
Using conc= moles÷volume
Conc of acid= 0.047 ÷ 0.038= 1.24 molecular per decimeters cubed

Atom economy= (164.1×3)÷ (164.1×3)+(98×2)
×100
71.5%

I might have slipped up somewhere but I'm sure my methods right

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7. For part c where exactly is the x ?
Can't tell from pic

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8. (Original post by coyote101)
For part c where exactly is the x ?
Can't tell from pic

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Thanks The x is just outside the bracket. I think it's 2 as the numbers inside the bracket are half the empirical formula but I might be wrong.

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