This means that for any sample of pure sodium bicarbonate, the fraction 0.274 of the mass of that sample is sodium, and 0.012 of the mass of the sample is hydrogen, etc. Exact equivalence statements are statements that are true for all time in all places. They also specify two ways of saying the same thing. A special sense equivalence statement may be true for all time, but the two materials which are related are not really the same things. Other special sense equivalence statements may be true only for given situation.
Speed is an example.
Suppose a car is moving 60 miles per hour. However they are the same in the sense that 1 hour's worth of travel in this car at this speed will result in 60 miles of distance covered. This will not be true if the car changes speed. Later we'll see density is also a kind of equivalence statement. Any equivalence statement can be converted into two unit factors which can be used to change one of the two quantities to the other quantity.