Question

If 100. mL of 0.400 Na2SO4 is added to 200. mL of 0.600 M NaCl, what is the concentration of Na+ ions in the?

Answer 1

First calculate the moles of Na+ contributed by each solution:

From Na2SO4 = 0.400 x 0.1 x 2 = 0.08 mol
From NaCl = 0.600 x 0.2 = 0.12 mol

Notice that for Na2SO4, I multiplied by 2 - this is because each molecule of Na2SO4 contains TWO atoms of Na, so for however many moles of Na2SO4 you have, you actually have TWICE as many moles of Na.

In total, we have 0.08+0.12=0.2 mol of Na+. Now we calculated the new concentration. Note that the volume now that the solutions have mixed is 0.1+0.2= 0.3 L.

Na+ concentration = 0.2 mol / 0.3 L = 0.67 M