4-1 Distinguish between masses of individual atoms and average masses

4-1-1. The mass of one atom of Mg-26 is closest to
A. 24.3 u
B. 24.0 u
C. 26.0 u
D. 12.0 u



4-1-2. In a ficticous world, element X has only two isotopes: ⁷⁷X with a mass of 77.0 u and ⁷⁸X, with a mass of 78.0 u. The relative abundance of the heavier isotope is 30.0%. The average mass of the X atoms is....
A.77 u
B.77.3 u
C.77.5 u
D.78 u

4-1-3. In a fictious world, element Z has only two isotopes: ³⁵Z with a mass of 35.00 u and ³⁷Z, with a mass of 37.00 u. What is the relative abundance of the lighter isotope if the average mass is 35.8u.
A.40.0%
B.50.0%
C.60.0%