Question

Find molecular masses of ${K_2}C{r_2}{O_7}$ and $C{r_2}{(S{O_4})_3}$ .

Answer 1

The molecular mass is the mass of a given molecule. It is measured in Daltons. Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quantity relative molecular mass, as defined by $IUPAC$ , is the ratio of the mass of a molecule to the unified atomic mass unit and is unitless.

Complete Step By Step Answer:
The molecular mass or molecular weight is the total mass of a compound. It is equal to the sum of the individual atomic masses of each atom in the molecule. It's easy to find the molecular mass of a compound with these steps:
Determine the molecular formula of the molecule.
Use the periodic table to determine the atomic mass of each element in the molecule.
Multiply each element's atomic mass by the number of atoms of that element in the molecule. This number is represented by the subscript next to the element symbol in the molecular formula.
Add these values together for each different atom in the molecule.
The total will be the molecular mass of the compound.
Molecular weight of ${K_2}C{r_2}{O_7}$ $=$ $2$ $\times$ (atomic mass of $K$ ) $+$ $2$ $\times$ (atomic mass of $Cr$ ) $+$ $7$ $\times$ (atomic mass of $O$ )
$= 2 \times (39) + 2 \times (52) + 7 \times (16)$
$= 78 + 104 + 112$
$= 294amu$
Molecular mass of $C{r_2}{(S{O_4})_3}$ $=$ $2$ $\times$ (atomic mass of $Cr$ ) $+$ $3$ $\times$ (atomic mass of $S$ ) $+$ $4$ $\times$ $3$ $\times$ (atomic mass $O$ )
$= 2 \times (52) + 3 \times [(32) + 4 \times (16)]$
$= 2(52) + 3 \times [32 + 64]$
$= 104 + 3 \times [96]$
$= 104 + 288$
$=392amu$

Note:
The molecular mass and relative molecular mass are distinct from but related to the molar mass. The molar mass is defined as the mass of a given substance divided by the amount of a substance and is expressed in $\dfrac{g}{{mol}}$ . The molar mass is usually the more appropriate figure when dealing with macroscopic quantities of a substance.