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# chem a-level help plz watch

1. A sample of hydrated calcium sulphate CaSO4.xH20 has a rfm of 172 , what is the value of x , how am i meant to sort this when i dont know the moles or mass of anything . Help plz
2. If it has a rfm of 172 then you know that of that 172, the CaSO4 makes up 40 32 (4*16)=136 of the 172. Then you just do 172-136 to get 36 and that's the total mass of the water you've got. The molecular mass of one water molecule is 1 1 16=18 and you have a total of 36, so 36/18=2. Therefore, the x value is 2
3. (Original post by Cyberen)
If it has a rfm of 172 then you know that of that 172, the CaSO4 makes up 40 32 (4*16)=136 of the 172. Then you just do 172-136 to get 36 and that's the total mass of the water you've got. The molecular mass of one water molecule is 1 1 16=18 and you have a total of 36, so 36/18=2. Therefore, the x value is 2
thanks for the reply , where do i start this from , the hydrated side or the anhydrous side
4. (Original post by Ray_Shadows)
thanks for the reply , where do i start this from , the hydrated side or the anhydrous side
You start by working out the rfm of the anhydrous side first because that's the mass that you know isn't going to be a multiple of the water mass if that makes sense xD
5. (Original post by Cyberen)
You start by working out the rfm of the anhydrous side first because that's the mass that you know isn't going to be a multiple of the water mass if that makes sense xD
yeh i get it , thanks
6. (Original post by Cyberen)
You start by working out the rfm of the anhydrous side first because that's the mass that you know isn't going to be a multiple of the water mass if that makes sense xD
what about the empirical formula method , do u know about that because thats the one they showed us in college
7. (Original post by Ray_Shadows)
what about the empirical formula method , do u know about that because thats the one they showed us in college
Oh yeah for that method then you would need to know how many moles of the substance and the masses/% by mass for that one to work. I guess the empirical formula method would be more likely to get examined on because it's more of a practical sort of calculation and the other way is quite quick and simple if you know the method. But yeah it's just knowing which method you should use based on the data which you are given
8. (Original post by Cyberen)
Oh yeah for that method then you would need to know how many moles of the substance and the masses/% by mass for that one to work. I guess the empirical formula method would be more likely to get examined on because it's more of a practical sort of calculation and the other way is quite quick and simple if you know the method. But yeah it's just knowing which method you should use based on the data which you are given
aha ok thanks very much

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