I need help with this chemistry question. How do I calculate it?

Question

30.0mL of 0.300 M H3PO4 is mixed with 90.0mL of 0.200 M KOH and the mixture evaporated to dryness. Which salt will precipitate out: K3PO4, K2HPO4, KH2PO4 ? Show calculations

Answer 1

I am thinking the middle one. K2HPO4

Calculate the amount of OH- and H+ ions by multiplying the respective molarities by the respective volumes.

For H3PO4.
(0.3M H3PO4)(30ml H3PO4)(3 mmol H+/1 mmol H3PO4) = 27 mmol H+ (mmol = millimole)

KOH:
(0.2M KOH)(90ml KOH)(1 mmol OH-/1 mmol KOH) = 18 mmol OH-

So 18 mmol of the H+ will combine (2 H's from each molecule) with 18 mmol of OH-, so there will be one H left in the salt.

Answer 2

H3PO4 + 3 KOH -> 3 H2O + K3PO4

A precipitation reaction results in water and a salt, the salt in this equation being: K3PO4

Answer 3

Mols(H3PO4) = (0.30 x 30 cm^3)/ 1000cm^3
- do - = 0.009 mols.

Mols(KOH) = (0.20 x 90 cm^3)/ 1000 cm^3
- do - = 0.018 mols

therefore twice as much KOH reacts with H3PO4

H3PO4 + 2 KOH = 2H2O + K2HPO4

The salt is K2HPO4

Answer 4

The H3PO4 is: 0.3X30/1000= 0.009mols.
the KOH is the double... 0.2X90/1000=0.018mols

so you will neutralize two H+ for a precipitate of

K2HPO3 (100%)

Answer 5

First calculate moles of OH-
90/1000 * 0.2 = 0.018 moles
According to the equation OH- + H+ = H2O 1 mole of OH- reacts with 1 mole of H+.
So 0.018 moles of OH- reacts with 0.18 moles of H+.

Calculate how many moles of H+ there are.
Every mole of H3PO4 has 3 moles of H+
H3PO4 = 3H+ + PO43-
Moles of H3PO4 = 30/1000 * 0.3 = 0.009 moles
So there are 3 * 0.009 moles of H+ = 0.027 moles H+

The 0.018 moles of OH- reacts with 0.018 moles of these H+ which is 2/3 of them, leaving 1/3 unreacted.
This means 2/3 of the H in H3PO4 are replaced by K leaving
K2HPO4.
Good luck
Nigel

Answer 6

do it yourself you lazy boy.

Answer 7

Dudes, what the hell?!?!?!?!?!?!