What is the difference between exact mass and molecular weight?
Answers
Answer:
Actually there is no difference between molecular weight and exact mass. We can say that they are based on two different system of measurement like kilogram (kg) and pound (lb) or meter (m) and foot (ft).
‘kg’ and ‘m’ is based on Metric system where as ‘lb’ and ‘ft’ are based on English system or Imperial system of measurement.
1 kg = 2.204 lb
Mass of a ball is 1 kg (say). Instead of saying this we can say mass of the ball is 2.204 lb. They both represent the same quantity :- mass of the ball.
In same way, weight of the molecule / atom can be expressed in atomic mass unit (amu or u) or actual/exact mass (i.e mass expressed in kg). If we express the mass of a molecule in amu then we call it molecular mass/weight.
exact mass of a C atom = 1.992×10^-26 kg.Atomic weight is the number of times an atom is heavier than 1/12 th of ¹²C. For eg. the atomic weight of O atom is 16 amu. It means that O atom is 16 times heavier than 1/12 th of ¹²C. And molecular weight is the sum of the weight of the atoms that make up the molecule. For eg. the molecular weight of water is 18 amu. It means that a water molecule is 18 times heavier than 1/12 th of ¹²C (exact mass of a water molecule = 3.0 × 10 ^ -26 kg).
exact mass of an O atom = 2.6566962 × 10^-26 kg.
exact mass of water molecule = 3.0 x 10 ^ -26 kg.
We see the exact/actual mass of the atoms/molecule is really tiny, which leads to some bothersome figures and is thus unfit for practical purpose. So we express it in atomic mass unit (it is also based upon a standard).