Question

A saturated solution of manganese(II) hydroxide,Mn(OH)2 , has a pH of 9.9. What is the hydronium ion concentration of the solution? Is the solution acidic or basic?

concentration=. M acidic or basic

What is the mass of solute, in grams, in 125 mL of a 1.068 x 10^-3 M solution of Na3PO4?

Mass of solute = g

What are the molar concentrations of the Na+ and PO4^3- ions?

Molar concentration Na+ = M Molar concentration PO4^3- = M

Answer 1

1)

pH = 9.9

[H+] = [H3O+] = 10^(-pH) = 10^(-9.9) = 1.25 * 10^(-10)

Since the Hydronium ion concentration is lesser than 1 * 10^(-7) (which is hydronium concentration of water), hence the solution will be basic

2)

Number of moles of solute = Volume of solution (in L) * Molarity of Na3PO4

=> 125 mL * 1L/1000 mL * 1.068 * 10^(-3)

=> 1.335 * 10^(-4) moles

Molar mass of Na3PO4 = 3 * Molar mass of Na + Molar mass of P + 4 * molar mass of O

=> 3 * 23 + 31 + 4 * 16

=> 164 gm/mol

Mass of solute = number of moles * molar mass = 1.335 * 10^(-4) mol * 164 gm/mol = 218.94 * 10^(-4) g = 0.0218g

On dissolution, one mole of Na3PO4 gives 3 moles of Na+ and one mole of PO4(3-)

Na3PO4 =============== 3Na+ + PO4(3-)

molar concentration of Na+ = 3 * molar concentration of Na3PO4 = 3 * 1.068 * 10^(-3) = 3.204 * 10^(-3) M

molar concentration of PO4(3-) = molar concentration of Na3PO4 = 1.068 * 10^(-3) M