molarity equation HELP?

Question

i asked questions yesterday but i really don't get this stuff because there is a variety of ways for the wording of these molarity equations and it's confusing! i just have two more...like please help me set it up step by step if u can.

what mass of K3PO4 is required to prepare 4.00 liters of 1.50M solution?

a solution of (NH4)2SO4 is to be prepared that is 2.25M in NH4+. What volume of solution can be produced using 50.0 g (nh4)2SO4?

and also....what is the answer to "how many liters of a 0.5M sodium hydroxide solution would contain 2 moles of solute?" i had that question a quiz and got it wrong!!\
thank you.

Answer 1

MM(K3PO4)=212 g/mol

Molarity = moles/ V

1.50 = moles / 4

moles= 4

4 mole(212 g/mol)=848 g



MM(NH4)2SO4=132 g/mol

50 g/ 132 g/mol=0.379 moles

(NH4)2SO4 >> 2NH4+ + SO42-

so 0.379 moles (NH4)2SO4 give 2(0.379)=0.758 moles NH4+

2.25 = 0.758 / V

V=0.337 L


0.5 M = 2 / V

V= 4 L

Answer 2

K3PO4
MW =3*39+31+4*16=212g

Solution 1.5 M =318G/l
and 4l = 1272g