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1st part of my question:

Q) The concentration of an NaCl solution is 1M. Express this in percent wt. by vol. unit.

Ans:

$$\text{In %(w/v) unit}=\frac{58.5\pu g}{1000\pu{mL}}\times \frac{100}{100}=5.85\ \pu{\%(w/v)}\tag{1}$$

$$\text{In (w/v)% unit}=\frac{58.5\pu g}{1000\pu{mL}}\times \frac{100}{100}=5.85\ \pu{\%(w/v)}\tag{2}$$

2nd part of my question:

Q) The concentration of an NaCl solution is 1M. Express this in ppm unit.

Ans:

$$\text{In ppm(w/v) unit}=\frac{58500\pu{mg}}{1\pu{L}}=58500\ \pu{ppm}\tag{3}$$

$$\text{In (w/v)ppm unit}=\frac{58500\pu{mg}}{1\pu{L}}=58500\ \pu{ppm}\tag{4}$$

My questions:

  1. Which of $(1)$ and $(2)$ is correct?
  2. Also, the unit of molarity is $\pu M$, or the unit of mass is $\pu {kg}$. Similarly, what is the unit in the first part of my question? Is it $\text{%(w/v)}$? Is $\text{%(w/v)}$ a unit just like $\pu{kg}$ and $\pu{M}$?
  3. Which of $(3)$ and $(4)$ is correct?
  4. Also, is it okay if I append/suffix (w/v) at the end of ppm? For example, if I say the concentration of NaCl is $58500\ \pu{ppm(w/v)}$ instead of saying that the concentration of NaCl is $58500\ \pu{ppm}$, will it be okay?
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    $\begingroup$ By not explicitly presenting own attempts to answer it, you put the question in closing danger. $\endgroup$
    – Poutnik
    May 8, 2022 at 13:50

1 Answer 1

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None of the notations, ppm (w/v) or (w/v) ppm, is a standard notation. I don't see this used much in analytical chemistry publications. Very rarely, you might see ppm (w/v) in some papers. If you are doing your own calculations, or writing a report, one should clearly explain how you are expressing concentrations. The extra label (w/v) is not needed because it is understood that for aqueous solutions it will be w/v (for convenience). For water, ppm by w/v and ppm by w/w are almost identical. The mg/ L approximation for ppm does not apply very well to solutions whose density is significantly different from 1. One should be careful about it.

The International Union of Pure and Applied Chemistry book '"Quantities, Units and Symbols in Physical Chemistry" says

"Deprecated usage Adding extra labels to ppm and similar symbols, such as ppmv (meaning ppm by volume) should be avoided. Qualifying labels may be added to symbols for physical quantities, but never to units.

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  • $\begingroup$ Thank you so much for your answer! Also, could you please tell me which of the statements $(1)$ and $(2)$ seem more correct to you? $\endgroup$ May 11, 2022 at 4:47
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    $\begingroup$ @tryingtobeastoic, 5.85 w/v % looks fine without brackets. $\endgroup$
    – AChem
    May 11, 2022 at 5:14
  • $\begingroup$ Understood. Also, should I write $$\text{In % w/v unit}=\frac{58.5\pu g}{1000\pu{mL}}\times \frac{100}{100}=5.85\ \pu{\% w/v}\tag{1}$$ or $$\text{In w/v % unit}=\frac{58.5\pu g}{1000\pu{mL}}\times \frac{100}{100}=5.85\ \pu{\% w/v}\tag{2}$$ $\endgroup$ May 11, 2022 at 5:39
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    $\begingroup$ 2 is better because you can read it as weight by volume percent. Don't worry about these minor things...there are no hard and fast rules as long as you are clear. $\endgroup$
    – AChem
    May 11, 2022 at 5:59

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