for a substance of known molar mass, the number of moles of a sample can be calculated from the mass of the sample.
Is is Always true, sometimes true, or false?

Question

for a substance of known molar mass, the number of moles of a sample can be calculated from the mass of the sample.
Is is Always true, sometimes true, or false?

anonymous · Answer

Let's rephrase this. Water has a known molar mass of 18g/mole. How many moles are in 2.2Kg?

$$1 mole/18g (H_2O) *2.2Kg * 1000g/1Kg$$

anonymous · Answer

It is Always True, molar mass can be used to determine the number of moles in a sample, as shown in the post above.