Rubidium is a soft, silvery-white metal that has two common isotopes: rubidium-85 and rubidium-87. The actual mass of rubidium-85 is 84.91179 amu. The atomic mass of rubidium is 85.47 amu. If the abundance of rubidium-85 is 72.17% and the abundance of rubidium-87 is 27.83%, what is the actual mass of rubidium-87

86.91757 amu

Explanation:

From the question given above, the following data were obtained:

Isotope A (rubidium-85)

Mass of A = 84.91179 amu

Abundance of A = 72.17%

Atomic mass of rubidium = 85.47 amu

Isotope B (rubidium-87):

Abundance of B = 27.83%

Mass of B =?

The actual mass of rubidium-87 can be obtained as follow:

Atomic mass = [(Mass of A × A%)/100] + [(Mass of B × B%)/100]

85.47 = [(84.91179 × 72.17)/100] + [(Mass of B × 27.83)/100]

85.47 = 61.28084 + (Mass of B × 0.2783)

Collect like terms

85.47 – 61.28084 = Mass of B × 0.2783

24.18916 = Mass of B × 0.2783

Divide both side by 0.2783

Mass of B = 24.18916 / 0.2783

Mass of B = 86.91757 amu

Thus, the mass of rubidium-87 is 86.91757 amu

i think it's c. i'm not sure but i think so.