Chemistry homework help

[nute]

Hoping someone here can help me. I know nothing about chemistry and my son is struggling with his chemistry homework. The question is - What mass of Calcium Hydroxide would be formed after 10g of water reacts with 10g of Calcium Oxide?

Anyone know how to do this, and more importantly are you able to explain it so that I can then understand it and explain it to my son. I have a reasonable science knowledge however I always liked physics rather than chemistry.

Be grateful for any help ....

[WUT]

http://www.bbc.co.uk/schools/gcsebitesize/science/...

http://www.bbc.co.uk/schools/gcsebitesize/science/... bbc gcse bitesize reacting masses

http://www.learnchem.net/tutorials/stoich.shtml

https://www.khanacademy.org/science/chemistry/chem...
the video may help you in the last link.

Edited by WUT on Thursday 18th January 19:27

Edited by WUT on Thursday 18th January 19:28

Edited by WUT on Thursday 18th January 19:32

[Eddie Strohacker]

This'll sort you out. The question is getting at the combination of calcium & oxygen & losing the hydrogen in the water. Atomic mass innit.

https://sciencing.com/calculate-mass-ratio-8326233...

[nute]

Wow, that was quick guys, thanks.

i had found the BBC site and was working through it. I think the crux of it is getting the equation for the reaction correct.

I'm not sure if CaO + H2O = Ca(OH2) is correct, isn't there an O missing?

[GliderRider]

The basic principle is that different atoms have different weights, so although Calcium Oxide (CaO) reacts with H20 to CaO + H2O ? Ca(OH)2 the weight of water used will not be equal to the weight of the Calcium Oxide.

Molecular Weights

Hydrogen = 1
Oxygen = 16
Calcium = 40
Calcium Oxide (CaO)= 40 + 16 = 56
Water (H20) = 1 + 1 + 16 = 18
Calcium Hydroxide (Ca(OH)2) = 40 + 16 +16 + 1 + 1 =74

From this you can see that if you had 56 grams of Calcium Oxide, it would only react with 18 grams of water (the rest probably turning to steam), resulting in 74 grams of Calcium Hydroxide

You now need to work out ratios, so (10 x 56) + (10 x 18) = (10 x 74) = 560 + 180 = 740 now divide through by 56 to give

(560/56) + (180/56) = (740/56) = 10 + 3.214 = 13.214

So I think the answer is 13.214grams

This website may help: https://www.lenntech.com/calculators/molecular/mol...

Note: in website in the link, in the box next to 'calculate', separate the CaO and the H20 with a full stop

Caveats
I last did chemistry at O level thirty seven years ago and got a C :-(
If any chemists come along, listen to them, not me!

I hope his chemistry teacher isn't a pistonhead...

Edited by GliderRider on Thursday 18th January 20:04

Edited by GliderRider on Thursday 18th January 20:06

[GliderRider]

nute said:

I'm not sure if CaO + H2O = Ca(OH2) is correct, isn't there an O missing?

The two needs to be to the right of the second bracket, then it all works out.

[Simpo Two]

Can't help with the chemistry because I always just failed it, but chemicals don't have Capital Letters.

[V8LM]

Calcium oxide = CaO
Water = H2O

A mole is a number (a very large number).

A mole of calcium atoms has a mass of 40 g
A mole of oxygen is 16 g
A mole of CaO has a mass of 56 g

A mole of hydrogen has a mass of 1 g
A mole of water has a mass of 18 g

The reaction of calcium oxide with water to form calcium hydroxide is

CaO + H2O -> Ca(OH)2

One molecule of calcium oxide reacts with one molecule of water to form one molecule of calcium hydroxide

One mole of CaO reacts with one mole of H2O to form one mole of Ca(OH)2


10 g of CaO is 10 / 56 = 0.179 moles
10 g of H2O is 10 / 18 = 0.556 moles

Because the 'stoichiometry' (how many molecules react with how many molecules to make how many molecules) is 1:1, there is an excess of water, so the 0.179 moles of CaO reacts with 0.179 moles of water to form 0.179 moles of Ca(OH)2

One mole of Ca(OH)2 has a mass of 74 g (as all the atoms of one molecule of CaO react with all the atoms of one molecule of H2O to form one molecule of Ca(OH)2 .i.e. no mass is made or lost)

0.179 moles of Ca(OH)2 has a mass of 13.21 g

13.21 g of Ca(OH)2 is made from 10 g of CaO and 3.21 g of H2O. 6.79 g of water remains.

Edited by V8LM on Thursday 18th January 20:55

[nute]

Thankyou so much guys, really appreciate your help.

Ill go through your answers and once i understand it i can explain it to my son.

Thanks again!!!

[nute]

Ok, I understand it now and so does he. He got half way but just couldn’t relate the molar mas to the ratio in the formula. We have both learned something tonight, thanks all for your help.

[Wiccan of Darkness]

Here goes. First start with the equation. You also need to add the physical states.

CaO(s) + H2O(l) --> Ca(OH)2 (aq)

The physical states are solid, liquid and aqueous solution.

The equation shows that 1 mole of calcium oxide reacts with 1 mole of water to give an aqueous solution of calcium hydroxide.

Now the numbers. The relative atomic masses of each element is different.

Calcium is 40.
Oxygen is 16
hydrogen is 1.

1 mole of calcium is 40 grams
1 mole of oxygen is 16 grams
1 mole of hydrogen is 1 gram.

Calcium oxide is 40g+16g so has a relative molecular mass of 56g

Water is 18g
Calcium hydroxide is 74g (40 + 16 +16+1+1)

This means that 56 grams of calcium oxide will react completely with 18 grams of water to produce 74 grams of calcium hydroxide.

Since there are 10 grams of each, we can work out how much of each will react.

10/56 (10 divided by 56) is 0.1786 moles. Therefore 56 g of CaO is 1 mole, 10g is 10/56 moles.

According to the equation, 1 mole of Calcium oxide will react with 1 mole of water.

So how much water is there? RMM of water is 18, and we have 10g of water.

10/18 (10 divided by 18) = 0.555 moles of water.

If there is only 0.17 moles of oxide but 0.55 moles of water, there is more water than is required, which is why the end result will be an aqueous solution. There's water left over.

Now for the reaction. 1 mole of oxide + 1 mole of water = 1 mole of hydroxide.

If there was only 0.1786 moles of calcium oxide, therefore there will be 0.1786 moles of hydroxide.

Calcium hydroxide (Ca(OH)2 has a relative molecular mass of 74.

But there will only be 0.1786 moles.

The mass of 2 moles is 2 x 74
The mass of 1 mole is 1 x 74
The mass of 0.1786 moles is 0.1786 x 74
Which is 13.2164 grams of Calcium hydroxide.

We could work out how much water remains. Since there was 0.55 moles of water in total, but only 0.1786 moles was used, the remainng water is

0.55 - 0.1786 = 0.3714 moles. Which is 6.685 grams of water.

Now the clever bit. There won't be 6.685 grams of water remaining.

This reaction is highly exothermic (it produces a st load of heat) there will be 6.685 grams of water remaining, in practice it will be less due to the enthalpy of the reaction.

This last bit will score extra brownie points, unless son is only 12 and then it'll be obvious he's had help

Copy and paste this in to a word document, print it out and have a read tonight in bed, it'll also be handy tomorrow to explain to son.

If there's any bits you don't understand, drop me a PM but this is a standard question from GCSE chemistry.

I hope you understand the explanation bit. Haven't seen what's been written above but it'll probably be correct, but what you needed was the explanation.

Apologies for the delay, got kitty issues. The cat is being a right pill tonight.

[V8LM]

0.55 - 0.1786 = 0.377 moles. Which is 6.786 grams of water. (Equal to the 10 g at the start less the amount used)


And there will be that amount remaining, just may not be remaining as liquid in the container.

Edited by V8LM on Thursday 18th January 22:56

[nute]

Glider, V8 & Wican - thanks again, yes I do understand it and so does he now. Dead simple once you grasp the way it works.