Can someone please help me figure this out? It is a simple question. Medal to best answer.

Rubidium has two naturally occurring isotopes, rubidium 85 (atomic mass = 84.9118 amu; abundance = 72.15 percent) and rubidium-87 (atomic mass = 86.9092 amu; abundance = 27.85 percent) Calculate the average atomic mass of rubidium.

86.0000 amu
85.9105 amu
8576.80 amu
85.4680 amu

Question

Can someone please help me figure this out? It is a simple question. Medal to best answer.

Rubidium has two naturally occurring isotopes, rubidium 85 (atomic mass = 84.9118 amu; abundance = 72.15 percent) and rubidium-87 (atomic mass = 86.9092 amu; abundance = 27.85 percent) Calculate the average atomic mass of rubidium.
 	
	86.0000 amu
	85.9105 amu
	8576.80 amu
	85.4680 amu

aaronq · Answer

It's a weighted average.

Multiply the mass of each isotope by it's percent abundance, then add these values together.

bohotness · Answer

(84.9118 x 72.15) + ( 86.9092 x 27.85 ) / 100 =85.47 amu

beccab003 · Answer

Okay. So, I would multiply 85 by 0.7215? And then -87 by 0.2785? etc.

beccab003 · Answer

oh okay. but wouldn't the 72.15 be 0.7215?

beccab003 · Answer

never mind. So the answer would be the last one?

beccab003 · Answer

@bohotness