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Leon Rogers

How to convert from w/w% to molarity?

Curtis Childress  Follow

Weight percent is essentially the same as (I arbitrarily picked grams as the mass unit):

$$\%~\mathrm{w/w~X} = \mathrm{g~X\over g~solution}$$

In order to get to the $\mathrm{mol\over L}$ units of molarity, you have to convert by multiplying by the solution density, $\rho$, and dividing by the molecular weight of $\mathrm X$, $M_\mathrm{X}$:

$$\mathrm{g~X\over g~solution} \times \stackrel{\left(1\over M_\mathrm X\right)}{\mathrm{mol~X\over g~X}} \times \stackrel{\left(\rho\right)}{\mathrm{g~solution\over L~solution}} = \mathrm{mol~X\over L~solution}$$

One key thing to remember is that $\rho$ is the density of the solution, not of the pure solvent. If you're working with dilute solutions (below $\sim\!100~\mathrm{mM}$ for aqueous) these two densities will be similar, but in concentrated solutions they will diverge.

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Herb Lison  Follow
, $c(\ce{X}) = n(\ce{X})/V(\text{solution})$ for clarity. Then you can also easily see that the density of the solution is $\rho(\text{solution}) = m(\text{solution})/V(\text{solution})$. Your dimensional analysis works fine though. (In your first equation you need to multiply the right hand side with $100\%$, otherwise youre using mass fraction.)More
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Emile de Groot  Follow
mass fractionMore
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Gordon Braddock  Follow
Id rather write More
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Jonathan Belsey  Follow
$w(\ce{X}) = m(\ce{X})/m(\text{solution})$, which is obviously not in percent. I also would call it More
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John Crooks  Follow
(amount) concentrationMore
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Chris Johnston  Follow
  1. Use the density of the solution to get the total volume of the solution.
  2. Then use the weight percent of solute to determine the amount of substance of the solute.
  3. Use the amount of substance of the solute divided by the volume to get molarity.

Molarity is relevant to our concept of chemistry and kinetics of reactions. However wt/wt% is frequently easier to use on an industrial scale.

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Daniel Iyamuremye  Follow

W/W% is just the same as a any percentage but now the difference is that let say that a concentrated solution contains 67% w/w ammonia that means that there is 67g of ammonia in 100g of the solution; now let us consider a question

A concentrated solution of aqueous ammonia is $28.0\%\text{w/w }\ce{NH3}$ and has a density of $\pu{0.899 g/mL}$. What is the amount concentration of $\ce{NH3}$ in this solution?

Converting $28.0\%\text{w/w}$ to ratio form gives us $28/100$ then you multiply it with the density $\pu{0.899 g/ml}$ for ammonia and $\pu{17.04 g}$ is an equivalent of one mole.

$$28/100 \times \frac{\pu{0.899 g}}{\pu{1 ml}} \times \frac{\pu{1 mol}}{\pu{17.04 g}} \times \frac{\pu{1000 ml}}{\pu{1ml}}$$

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