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.

Relative atomic mass of the element is 63.6 amu.

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:

Fractional abundance of isotope 1 =

Fractional abundance of isotope 2 =

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

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

Average Atomic Mass of given element = 63.618 amu

Converting this into 3 significant figures, we get

Average atomic mass = 63.6 amu

answer: the enthalpy of the overall chemical reaction is +87.88 kj or +88 kj.

according to hess's law, the overall enthalpy of a reaction is the difference between the sum of enthalpy values of all the products and the sum of enthalpy values of all the reactants.

δh(reaction) = ∑nδh(products) - ∑nδh(reactants)

δh(reaction) = [δh +δh] - [δh]

δh(reaction) = [1(-287.02) + 1(0) ] - [1(-374.9)] kj = +87.88 kj

here, δh (pcl3) = -287.02 kj, δh(cl2) = 0, δh(pcl5) = -374.9 kj

therefore, enthalpy of the overall reaction is +87.88 kj or +88 kj.