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I have been performing haloform reactions using sodium hypochlorite. The most available source is common household bleach, which is a solution of $\ce{NaClO}$ in water. The bottles invariably give a concentration in unitless percentages: e.g., "$6\%$ Bleach."

I have assumed this to mean: (mass of solute) / (mass of solution) * 100.

Now I see that there are other (mathematically erroneous) uses of the percent symbol %, including a measure called WV (weight volume), which seems to be normalized by dividing gram weights of solute by 100 ml.

An easy question, impossible without the units:

How many grams of $\ce{NaClO}$ are in one litre of $6\%$ household bleach?

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I have assumed this to mean: (mass of solute) / (mass of solution) * 100

Most likely you are correct in this assumption. Percentage compositions of ionic solutes in water are nearly always implicitly expressed in percent-by-weight ("% w/w").

$6\%$ bleach is relatively dilute and will have a density very near to $1\ \frac{\mathrm{g}}{\mathrm{mL}}$. So, $1\ \mathrm{L} \approx 1\ \mathrm{kg\ solution}$, and thus one can expect to have approximately $60\ \mathrm{g}\ \ce{NaClO}$ per liter of such a solution.

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