Answer is: the average atomic mass of chlorine is 35.4548.
m(³⁵Cl) = 34.97 amu; the average atomic weight of chlorine-35.
m(³⁷Cl) = 36.97 amu; the average atomic weight of chlorine-37.
ω(³⁵Cl) = 75.76% ÷ 100% = 0.7576; fractional abudance of chlorine-35.
ω(³⁷Cl) = 24.24% ÷ 100% = 0.2424; fractional abudance of chlorine-37.
m(Cl) = m(³⁵Cl) · ω(³⁵Cl) + m(³⁷Cl) · ω(³⁷Cl).
m(Cl) = 34.97 amu · 0.7576 + 36.97 amu · 0.2424.
m(Cl) = 35.4548 amu; average atomic mass of chlorine.
The unified atomic mass unit (amu) is a standard unit of atom mass.
One unified atomic mass unit is approximately the mass of one nucleon (1.66·10⁻²⁷ kg).