Question

What is the concentration of a 5.0 M Na3PO4 solution whose volume is doubled by added water? Wanna know? Make up a volume calculate the number of moles in that volume if it's 5.0 M and then put that number of moles in twice the volume and recalculate the new molarity.

250. mL of a 5.00 M stock solution is diluted to a final volume of 1.25 x 103 mL. What is its concentration in molarity?

Calculate the final mass in grams of Na3PO4 in a 5.00 M stock solution that is diluted from an initial volume of 25.0 mL to a final volume of 67.5 mL. Here's how you do it: Original concentration times its volume = number of moles present times F.W. = number of grams. These are also present in the diluted solution since you didn't add any more of the analyte, just water.

Answer 1

When a solution is diluted Number of moles in solution remains conserved (Moles of intial solution = Moles of final solution).

Number of moles = Molarity of the soluton * Volume(in L)

• When 5.0 M Na3PO4 solution volume is doubled by added water - Let intial volume of the solution be V. Moles of Intial solution = 5*V. When solution volume is doubled that means its volume is 2V. Let Molarity of final solution be x. So Moles of Final solution = x*2V. Moles of intial solution = Moles of final solution ⇒ 5*V = x*2V ⇒ x = 2.5M. So concentration of final solution is 2.5M.
• When 250. mL of a 5.00 M stock solution is diluted to a final volume of 1.25 x 103 mL - Moles of Intial solution = 5*0.25= 1.25. Let Molarity of final solution be x. So Moles of Final solution = x*1.25. Moles of intial solution = Moles of final solution ⇒ 1.25 = x*1.25 ⇒ x = 1M. So concentration of final solution is 1M.
• When 5.00 M stock solution that is diluted from an initial volume of 25.0 mL to a final volume of 67.5 mL - When we dilute the solution, number of moles remain conserved, therefore Moles in final solution = Moles in intial solution = 5*0.025 = 0.125 moles. Mass of Na3PO4 = Moles * Molecular weight of Na3PO4 = 0.125 * 163.94 = 20.4925g. So final mass of Na3PO4 is 20.4925g.