What is the mass number of a carbon isotope that has seven neutrons

Explanation:

Eqn of Reaction

H2 + Cl2 == 2HCl

Mass of HCl=72.9g

Mass of H2 = 2.0g.

Moles of HCl = Mass/Molar Mass

n = 72.9/36.5

n=1.997 moles of HCl was produced.

Now from the balanced eqn of reaction...

2moles of HCl is produced when 1moles of Cl2 reacts.

So

1.997moles of HCl would produce

1.997 x 1/2 moles of Cl2

0.999moles of Cl2 is gotten.

n(Cl₂) = mass/Mm

Mass = Mm x n

n=0.999moles

Molar Mass(Mm)= 35.5 x 2= 71g/mol ( You multiply by 2 cuz Chlorine is a diatomic gas).

Mass = 0.999 x 71

Mass = 70.929g of Chlorine.

An easier and swifter way to look at this is

From the Law of Conservation Of Mass.

The Mass of reactants MUST equal those of products in a simple chemical reaction since Matter is neither created nor destroyed in this scenario.

So The product Was 72.9g of HCl

Therefore

The Sum of both reactants Must equal this value.

We're already given the value of H₂ =2g

Therefore...

Chlorine should be undoubtedly 70.9g.

Hope this helps.

Have a great a day Amigo✅