Question

Doubts in stoichiometry/mole concept?

I have just started learning chemistry. In many problems that I came across, it seems to be using the following fact:

Summation of (no. of moles of the acid in a mixture × basicity of the acid) = Summation of (no. of moles of the base × acidity of the base). Is this correct and in any way related to the law of equivalence?

For example this problem: A mixture of three acid solutions : 50ml of 0.1M HCl, 10ml of 0.05M HNO3 and 'V' ml of 0.05M H2SO4 for complete neutralization of 150ml of 0.1M NaOH solution. Calculate the value of 'V'ml.

I understand the reactions each acid undergoes, but I didn't understand this step of the solution:

1×Number of milli moles of HCl + 1×Number of milli moles of HNO3 + 2×Number of milli moles of H2SO4 =
1×Number of milli moles of NaOH.

Can someone please explain me this?

Answer 1

The simple equation you are working with is this:
`H^+ + OH^- = H_2O`
See Below

Explanation:

The thing about those acids is that they are all strong acids. That means they totally give away their `H^+` ion. If you have 12 HCl, you'll have 12 `H^+`, etc.

`H_2SO_4` is the one that given `H^+`.

So you need the number of `H^+` from `HCl`, `HNO_3`, and `H_2SO_4`. Once you have these, then you need the same number of `NaOH`.

It is like bringing a dog to a dog park. I'm the guy (`OH^-`) who picks the poop up (`H^+`, and you and your 2 friends bring dogs. You bring 2 dogs, your other friend brings 1 dog, and the other brings 1 dog. Each dog poops (`H^+`)....and so i've got 4 poops to pick up.

So, you calculate the number of moles (or millimoles...i'd recommend doing moles till you get the concept cemented in your head) for each acid and this will be the number of moles of `H^+`. That will just be equal to the moles of `NaOH`

Molarity = `("moles")/("liter")`
So:
`M_"HCl"xxV_"HCl" + M_"HNO3"xxV_"HNO3" + 2x(M_"H2SO4"xxV_"H2SO4") = "total moles" H^+`

`"total moles" H^+ = M_"NaOH"xxV_NaOH = "total moles" OH^-`