[chestudent]

If I have measured a sample taken out of a 8% w/v suspension, and the mass of sample equals .03 grams, what is the mass of the solute in the sample? The suspension is made of polystyrene dispersed in water. The density of polystyrene is 1.055 g/mL.

My approach: Since the sample is .03 grams, I found the volume = 375 microliters, using w/v specification.

I also tried using: .08 = (1.055 g/mL * Volume PS)+ (1 g/mL)*(volume water)

but I seem to be missing some information.

[Hunter2]

If 8% is your sample mass/volume then 92% is the solvent mass/volume.

You have to calculate first the mixed specific gravity . The single ones both are given. With this value you can convert to the volume of the sample. And 92% from this will give your mass of solvent.

[DrCMS]

If I have measured a sample taken out of a 8% w/v suspension, and the mass of sample equals .03 grams, what is the mass of the solute in the sample? The suspension is made of polystyrene dispersed in water. The density of polystyrene is 1.055 g/mL.

My approach: Since the sample is .03 grams, I found the volume = 375 microliters, using w/v specification.

Check your maths again that volume is not correct for that mass of a mainly water solution.

[chestudent]

DrCMS:

Isn't .03 g/ (.08 g/ mL) = .375 mL = 375 microliters? The sample mass measurement may be off, but I am trying to understand the concept.

Or is the one below, the correct approach?

Hunter2:
Mixed specific gravity----
(1.055 g PS/ mL PS)(.08) + (1g H2O/ mL H2O)(0.92) = 1.0044 g solution/ mL solution

.03 g solution/ (1.0044 g solution/ mL solution) = .0298 mL solution (this is the sample volume)

Mass of solute in sample = (.0298 mL solution) * (.08 g PS/ mL solution) = .00238 g PS

[mikasaur]

Your approach using Hunter2's hint looks good.

Your first one won't work. Do you see why? You're doing a pretty good job of including units in your calculations (for which you should be applauded), but in your first approach it's .03 g of what? It's .08 g of what? Are they the same things?

[chestudent]

Yes, I understand. The first approach is assuming that the mass measured is of just the solute, when it's really solute + solvent.

0.03 g of PS+water sample.
0.08 g PS/ mL water is the w/v % of the polystyrene suspension. (see problem statement)