I have a problem which I solved with two different methods. I don\'t understand

Question

I have a problem which I solved with two different methods. I don't understand why the final answer differs from one method to the other because logically they should be the same. Can anyone point out the difference / mistake in any of these, and specify which is the correct method to use in the case where one of the two methods I used is wrong/should not be used.

So the problem is: Using solid Na2SO4, describe the preparation of 9.00L of a solution that has a concentration of 60 ppm Na+.

These are the two methods I used.

Explanation / Answer

Since density of water 1kg/L
mass of water = 9 Kg

ppm = mass of solute *10^6/mass of solution
60 = mass of Na+ *10^6 /9
mass of Na+ = 5.4*10^-4 gm

number of moles of Na+ = 5.4*10^-4 /23 = 2.35*10^-5 mol

so, number of moles of Na2SO4 = 2.35*10^-5 /2 = 1.18*10^-5 mol

Molar mass of Na2SO4 = 142.04 g

so mass of Na2SO4 = 142.04 * 1.18*10^-5 = 1.67*10^-5 g

So mass is 1.67*10^-5 g = 167 mg

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.