# Chemistry A level helpWatch

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#1
Can someone guide me on how to do these questions, I have looked online and cannot find a decent explanation on how to do these questions. Thanks

1. Calculate the number of molecules of water in 150 g of hydrated copper sulfate - CuSO4•5H2O
2. Calculate the number of molecules in 1 cm^3 of water (density of water = 1 g cm^−3)
3. The number of atoms of helium in a balloon with a volume of 5 dm^3
(assume the density of helium to be 0.17 g dm^−3)
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#2
Bump
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2 years ago
#3
For the first question you would first find out the molar mass of the hydrated copper sulfate by using the periodic table.
Once you have found the molar mass divide it by the 150g so mass/mr = Moles of Hydrated copper sulfate.
Then to isolate the number of water molecules, you do a ratio of the answer to 18 and you should get a number.
I hope this makes sense and gives you the right answer.
1
2 years ago
#4
(Original post by Arayan01)
Can someone guide me on how to do these questions, I have looked online and cannot find a decent explanation on how to do these questions. Thanks

1. Calculate the number of molecules of water in 150 g of hydrated copper sulfate - CuSO4•5H2O
2. Calculate the number of molecules in 1 cm^3 of water (density of water = 1 g cm^−3)
3. The number of atoms of helium in a balloon with a volume of 5 dm^3
(assume the density of helium to be 0.17 g dm^−3)
sorry you do not do a ratio with 18. Instead multiply the answer (from dividing the mass/mr) by 5.
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2 years ago
#5
Q2: There is a volume of 1 cm^3 of water, and as we are given the density we know also that this is equal to 1g of water. Using the equation moles = mass/molar mass, use this mass and divide it by the molar mass of water (18gmol^-1). This gives you the amount of moles of water. Now as we know that 1 mole has Avagadro's Constant (6.022x10^23) molecules, so we multiply the moles of water by Avagadro's constant.
Q3 is the same principle, except the density is not the same as the volume, so mass will be different.
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#6
Thanks guys, solved it!!
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