Question

You add 20 mM of acetate (CH3COOH), 10 mM of HCl, and 40 mM of KHCO3 into
100 mL of pure water and immediately cap it with a pure N2 headspace (with 100 mL of volume).
At equilibrium what is your total chemical species and pH of the water. pKa of acetate is 4.74.
Don’t forget KH, K1, K2 of the carbonate system!

Answer 1

I don't understand the question.
1. If mM stands for millimolar, then HOW MUCH (what volume) of these solutions is being added. Perhaps mM stands for millimoles?
2. You say acetate is being added so which acetate is being added. Or are you adding aceti acid?

Answer 2

100ml of water
mM is millimoles
20 mM of acetate is being added along with 10 mM of HCl, and 40 mM of KHCO3