Hello, I have a question that is related to % w/v. The question is as follows:

You have a 20% w/v stock solution of a sugar of molecular weight 160 Daltons. How much of this stock solution will you need to use to...

**a. prepare 10 ml of solution at 350 mg sugar per ml.**

Attempt:

So 20% w/v stock solution = 0.2 g/mL

350 mg/mL x 10mL gives us 3500mg of the sugar, or 3.5g

so the amount of solution we want is

x mL = 3.5 g / (0.2g / mL) = 17.5mL ( is this correct?)

**b. 7 ml of solution containing 15 micromoles of sugar**

Attempt:

15umol = 1.5 x 10^-5 mol

1.5 x 10^-5 mol (160 Daltons / 1mol) = 2.4 x 10^-3 g

(2.4 x 10^-3g) / 7mL = 4.89 x 10^-5 g/mL

and then I am stuck. How are you supposed to make 4.89 x 10^-5g/mL solution from a 0.2g/mL stock solution?! Is it possible? it does say in the question If the specific solution cannot be made, explain why not.

thank you in advance!