Why is mass % used more commonly than molarity?

Question

I need to know why mass % or weight % is more commonly used on commercial products than is molarity?

Not really sure why at all, other than maybe people don't remember what the heck molarity is.

Anyway, and chemistry-related answers would really be appreciated. Thanks! (+10 for best answer)

Answer 1

Some units of concentration -particularly the most popular one (molarity)- require knowledge of a substance's volume, which -in contrast to mass- is the variable depending on ambient temperature and pressure. In fact (partial) molar volume can even be a function of concentration itself. This is why volumes are not necessarily completely additive when two liquids are added and mixed. Volume-based measures for concentration are therefore not to be recommended for non-dilute solutions or problems where relatively large differences in temperature are encountered

Answer 2

The problem with molarity is that it changes with temperature. Say you make a 1M solution of NaCl at 20 degrees C. If you heat it up to 50 C, the volume of the water increases, which means the molarity decreases, even though it is the same mass of NaCl in the same amount of water.

% by mass does not change with temperature, so it is slightly more accurate and used on some commercial products.