Question

Gram molecular mass is:
(a). The mass of a molecule expressed in grams.
(b). Same as molecular mass where unified mass (u) is replaced by grams.
(c). The molar mass of a molecule in grams.
(d). Sum of the atomic mass of all atoms of that molecule.

Answer 1

- Hint: Recall the definition of gram molecular mass,
For example, Molecular mass of oxygen is 32 amu and thus,
1 gram molecule of oxygen = gram molecular mass of oxygen = 32 g.

Complete step-by-step solution -

Gram molecular mass is the molecular mass of a substance (in amu) expressed in grams or it is the mass in grams which is numerically equal to its molecular mass.
It is the same as the molar mass. The only difference is in gram molecular mass unit of mass has to be specified.
How to Find Gram Molecular Mass-
We should use the molecular formula to calculate molar mass.
Look up the relative atomic mass of each element in the formula. Suppose \[{{A}_{x}}{{B}_{y}}{{C}_{z}}\] is molecular formula of a compound where A, B & C are the atoms; x, y & z are the no. of atoms used respectively. Now if atomic no. of A, B & C are a, b & c respectively, then,
Multiply the subscript after each element symbol (the number of atoms) by that element's atomic mass. If there is no subscript, it means there is only one atom of that element in the molecule.
Add all of the values together to find the gram molecular mass.
That is-
Gram molecular mass of \[{{A}_{x}}{{B}_{y}}{{C}_{z}}\] is-
\[(a\times x)+(b\times y)+(c\times z)g\]
For example: In $H_2SO_4$; 2 no.s of hydrogen molecule, one sulphur molecule and 4 no.s of oxygen molecule are present.
So, the gram molecular weight of $H_2SO_4$ is
\[(1\times 2)+(32\times 1)+(16\times 4)g=98g\]
[Atomic weight: H=1, O=16, s=32]
Therefore, gram molecular mass is the same as molecular mass where unified mass (u) is replaced by grams and thus option B is the correct one.

Note: We use gram molecular mass to calculate gram moles. On the basis of the definition of gram molecular mass the mass of a substance is related to its number of moles according to the relation:
\[Grammoles=\dfrac{massingrams}{Gram Molecular Mass}\]