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# Acids, bases and salts calculation help watch

1. Hi,
Was set these calculation questions for chemistry and not sure how to go about working them out. If anyone could give me any advice without the answer it would be much appreciated

What volume of 2M HCl would be required to neutralise 20cm3 of 0.5M sodium hydroxide.

45cm3 of 0.2M sodium hydroxide just reacts with 10cm3 of 0.3M phosphoric acid. Use this data to show that the formula of phosphoric acid is H3PO4
2. H+ + OH- <=> H2O
What do you get when you put sodium hydroxide in water, and HCl in water etc.
need help with the actual calculations though like what equations should I use?
4. Give me 5 minutes, I think I know how to do the first one
5. So the reaction equation is HCl + NaOH -----> NaCl + H2O

What volume of 2M HCl would be required to neutralise 20cm3 of 0.5M sodium hydroxide.

First, find the number of moles of sodium hydroxide (in dm3)
There are 1000cm3 in 1 dm3
So, 20cm3= 20/1000dm3
= 0.02dm3

No. of moles= conc* voulme
No. of moles= 0.5*o.o2
No. of moles = 0.01

According to the equation, if there are 0.01 moles of sodium hydroxide, there must also be 0.01 moles of HCl

Volume= no. of moles/concentration
Volume= 0.01/2
Volume= 0.005 dm3
So, in cm3, the volume will be 0.005*1000
So, the volume of HCl needed is 5cm3

6. With the first one you calculate the number of moles of NaOH first
You know that for neutralisation to occur, the moles of the acid and base must be equal (since they are monoprotic and monibasic) so the moles of acid required must be the same
And then rearrange the volume x concentration = moles to find the volume (as you are given concentration and have just worked out moles)

For the second one you just calculate the number of moles of phosphoric acid and NaOH
You will find that there are 3 times LESS moles of phosphoric acid
This means phosphoric acid must be able to donate 3 protons (have 3 hydrogen atoms)
This gives you the H3 bit and since PO4 gives a 3- charge then it must be H3PO4

I think this is right but I haven't used any resources and did it fairly quickly so don't take my word for it haha

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7. Thank you both!

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