 admin # Element X is found in two forms: 90.0% is an isotope that has a mass of 20.0, and 10.0% is an isotope that has a mass of 22.0. What is the atomic mass of element X? A. 20.0 B. 20.2 C. 20.8 D. 21.2 E. 21.8 Can anyone explain this one for me?

2 months ago

## Solution 1 Guest #9847794
2 months ago
$$M_{X} = \frac{(20.0\cdot 90\%)+(22.0\cdot10\%)}{100\%} = \frac{2020}{100} = 20.2[u]$$

## Solution 2 Guest #9847795
2 months ago

The relative atomic mass of element X which has 2 isotopes  is 20.2 u.

### What is relative atomic mass ?

Relative atomic mass of an element is an average of masses of atoms of isotopes. It is found out by the formula ,

relative atomic mass=(mass×abundance of isotope 1 )+(mass×abundance of isotope 2)/100

Considering the example given ,atomic mass of element X is found out as,

relative atomic mass=(20×90)+(10×22)/100=2020/100=20.2 u.

Relative atomic mass is a dimensionless physical quantity  which is an average  ratio of mass of atoms in a sample to the atomic mass constant.Atomic mass constant is 1/12 of mass of carbon-12 atom.

Average atomic mass is relative atomic mass  which is calculated from percent composition.Relative atomic mass is unit less and is a physical quantity.It is a multiple of standard atomic mass unit.

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