Question

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...

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