Question #8b2b6

We usually reserve parts per million to express very, very small concentrations of solute, sometimes called trace amounts, in a given solution, but you can pretty much use parts per million to express any concentration if you want.

A concentration of #"1 ppm"# corresponds to #1# part solute present for every #10^6# parts solution. To find a solution's concentration in parts per million, you can use the equation

#color(blue)(ul(color(black)("ppm" = "grams of solute"/"grams of solution" xx 10^6)))#

If you take water's density to be approximately equal to #"1.0 g mL"^(-1)#, then you can say that your solution will contain

#1000 color(red)(cancel(color(black)("mL"))) * "1.0 g"/(1color(red)(cancel(color(black)("mL")))) = "1000 g"#

After you mix the solute and the solvent, you will end up with

#m_"solution" = "400 g" + "1000 g" = "1400 g"#

This means that the solution will have a concentration of

#(400 color(red)(cancel(color(black)("g"))))/(1400color(red)(cancel(color(black)("g")))) xx 10^6 = color(darkgreen)(ul(color(black)(3 * 10^5color(white)(.)"ppm")))#

The answer is rounded to one significant figure.

As you can see, it's not very practical to use parts per million for such concentrated solutions.