Find the molar mass of 30Si if the average molar mass of Si is 28.09 g/mol?

Silicon has three naturally occurring isotopes (28Si, 29Si, and 30Si). The molar mass and natural abundance of 28Si are 27.98 g/mol and 92.20%, respectively, while the molar mass and natural abundance of 29Si are 28.98 g/mol and 4.67%, respectively. I'm really stuck on how to solve this, it's a practice final exam question for chemistry ...

Question

Gracie Figueroa

Chemistry

25 October, 23:07

Find the molar mass of 30Si if the average molar mass of Si is 28.09 g/mol?

Silicon has three naturally occurring isotopes (28Si, 29Si, and 30Si). The molar mass and natural abundance of 28Si are 27.98 g/mol and 92.20%, respectively, while the molar mass and natural abundance of 29Si are 28.98 g/mol and 4.67%, respectively. I'm really stuck on how to solve this, it's a practice final exam question for chemistry ...

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Answers (1)

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Hahn · Answer 1

First let's find how much 30Si in nature: 100%-92.2%-4.67%=3.13%

Average M = M (28Si) * 0.922 + M (29Si) * 0.0467 + M (30Si) * 0.0313;

Let's substitute all the values we know:

28.09 = 27.98*0.922 + 28.98*0.0467 + x * 0.0313;

x = 30.03 g/mol