Different Units
Ratios are unitless when they relate quantities in units of the same dimension.
For example, the ratio 1 minute : 40 seconds can be reduced by changing the first value to 60 seconds. Once the units are the same, they can be omitted, and the ratio can be reduced to 3:2.
In chemistry, mass concentration "ratios" are usually expressed as w/v percentages, and are really proportions.
For example, a concentration of 3% w/v usually means 3g of substance in every 100mL of solution. This cannot easily be converted to a pure ratio because of density considerations, and the second figure is the total amount, not the volume of solvent.
Read more about this topic: Ratio
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