Question #a1ef4

1 Answer
Stefan V.
Oct 27, 2015

"31.3 g"

Explanation:

Start by making sure that you have clear understanding of what a "25%w/v" sodium chloride solution looks like.

A weight by volume solution is defined as mass of solute, which in your case is sodium chloride, divided by the volume of the solution, and multiplied by 100.

color(blue)(%"w/v" = "mass of solute"/"volume of solution" xx 100)

This means that a 25%"w/v" sodium chloride solution would contain "25 g" of sodium chloride for every "100 mL" of solution.

You know that you dissolve the sodium chloride in "125 mL" of water, so that will be your final solution's volume.

If you know the target weight by volume concentration, and what the volume of the solution is, you can solve the above equation for the mass of sodium chloride

m_"solute" = (%"w/v" xx V_"solution")/100

m_"solute" = (25"g"/color(red)(cancel(color(black)("mL"))) * 125color(red)(cancel(color(black)("mL"))))/100 = "31.25 g"

I'll leave this rounded off to three sig figs, just for good measure

m_"NaCl" = color(green)("31.3 g")

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