# Homework Help: Question regarding concentration units

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1. Jul 25, 2016

### Lassy

Hi all,

I've done some experiments and I'm trying to work out the correct unit of my final concentration. The experiment goes thus far:
I added 800 uL of a substance into a litre of my sample (800uL/L).
This substance has a density of 1020 kg/m3 and is 60% w/w.
I'm trying to work out the amount of substance per litre of sample.
After my calculations I have arrived at a number of 490 but I will appreciate some clarification on this as well as if the correct final unit of concentration will be ug/L or mg/L.

2. Jul 25, 2016

### Staff: Mentor

800 $\mu L$ = 0.8 cc

1020 kg/m3 = 1.02 gm/cc

(0.8)(0.6)(1.02) = 0.49 gm = 490 mg

3. Jul 25, 2016

### Lassy

I have one more question, what if the density was 1.020 kg/m3?

Will the final concentration still be in mg/L or ug/L?

4. Jul 25, 2016

### Staff: Mentor

I moved the thread to our homework section.

As long as you keep your units consistent, that problem never comes up.

1.020 kg / m3 * 1m3/(1000 L) * 800 µL * 0.6 = ..

5. Jul 25, 2016

### Staff: Mentor

That's a pretty low density

6. Jul 25, 2016

### Lassy

Yes, but that's what is quoted on the data sheet for the substance.

This is the calculation I did:

Mass = D x V = 1.02mg / mL x 0.8mL / 1000 mL = 0.000816 mg/mL = 0.816 mg/L = 816 ug/L
816 ug/L x 60% = 490 ug/L

7. Jul 25, 2016

### Staff: Mentor

So, where did the 1020 kg/m3 come from? This is 1.02 gm/cc.

8. Jul 26, 2016

### Staff: Mentor

1.020 kg/m3 is a density lower than air. There is no liquid with that density.

Could the source be from some foreign country? Compared to English, some other languages have a swapped use of "," and "." for numbers (e.g. 12345.6 can be written as 12.345,6), so 1.020 could mean 1020.