Question

In a mass spectrum, there are two peaks, one at m/z 63.0 with percentage abundance 69.1%, and another peak at m/z 65.0 with percentage abundance 30.9%. Determine the relative atomic mass (Ar) for this element, giving the answer to 3 significant figures.

Answer 1

Answer: Relative atomic mass of the element is 63.6 amu.

Step-by-step explanation: We are given an element having 2 peaks in the mass spectrum, which means that the masses of two isotopes of an element are given.

m/z value of isotope 1 or mass of isotope 1 = 63amu

m/z value of isotope 2 or mass of isotope 2 = 65amu

Fractional abundance of the isotopes can be calculated as:

`\text{Fractional abundance}=\frac{\%\text{ abundance}}{100}`

Fractional abundance of isotope 1 =
`(69.1)/(100)=0.691`

Fractional abundance of isotope 2 =
`(30.9)/(100)=0.309`

To calculate average atomic mass of an element, we use the formula:

`\text{Average atomic mass}=\sum_(i=1)^n(\text{Mass Number})_i* (\text{Fractional Abundance})_i`

Now, putting the values of abundances and mass numbers of 2 isotopes in above equation, we get

`\text{Average atomic mass}=(63* 0.691)+(65* 0.309)`

Average Atomic Mass of given element = 63.618 amu

Converting this into 3 significant figures, we get

Average atomic mass = 63.6 amu