How does % purity on household products work?

For example, toilet bowl cleaner is 9.5% HCl. Does this mean that for every 90.5 moles of H2O (or other stuff) there is 9.5 moles of HCl? Does it mean that for every 90.5 g of H2O there are 9.5 g of HCl? 





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> For example, toilet bowl cleaner is 9.5% HCl.
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First off, do you see ["*w/v*"](https://en.wikipedia.org/wiki/Mass_concentration_(chemistry)) mentioned after those figures, i.e- $\ce{HCl}$ 9.5% (*w/v*)?



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> Does this mean that for every 90.5 moles of H2O (or other stuff) there is 9.5 moles of HCl? Does it mean that for every 90.5 g of H2O there are 9.5 g of HCl?
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Even if you don't, I'd still expect the concentration to be expressed in the form: $x$ grams of $\ce{HCl}$ for every $y$ units of water. As for whether water's mass or volume's considered, it would depend on the regulations for such products (which I'm unfamiliar with).


However, I wouldn't be surprised if it *did* turn out to be a *w/v* quantity (which is quite common in pharmaceutics).



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> Kinda weird that we don't know if this is weight or molarity.
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Like I said, I'm going to throw my weight behind "weight", since it's *far* more common than molarity outside of a lab (come to think of it, I've never seen a household product's ingredient concentration expressed in molars).


Not exactly important, but since you mentioned "molarity", here's a handy formula to convert something in % *w/v* to molars:


$$\text{Molarity} = \frac{{10}{(w/v)}{(d)}}{M}$$


Where $\ce{d}$ is the density of the solution, $\ce{M}$ is the molar mass of the solute and $\ce{w/v}$ is the mass concentration of the solute.


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I would take it here and in general as mass percentage. Expressing conc as % always leads to doubts although it is originally intended as mass to mass ratio. The good practice should be stating m/v when is the case. 


Household HCl per se (not other ingredients) should be at about the percentage you mention in m/m.


Note that as always with solution in water, the concentration expressed in the two ways tend to the same value as lower it is. That is the reason people started to neglect the difference.


I am sensitive to this because in my domain (interdisciplinary) you face concentration given as % with no more info and the solvents are even organic !


So that the actual concentration or two supposedly equally concentrated samples can be very different. This leads to ambiguity in protocol but can highly interfere with data interpretation and relationships. Fortunately it is nothing about quantitative analysis. 


Basically you are let in dark and (I do, based on experience) guess they mean 1 mg for 1 ml solvent ! 


I mostly write this to sensitised the readers to adopt the good practice. 1 mg of matter in 1 ml of dichloromethane is 1 mg/ml, not 1%! Alternatively it does not cost much to add m/v.


Edit: not to mention that v should refer to the final volume of the solution. ...


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