Let me explain this in a simpler way.
As we know,Z Of carbon=12
So,6.023×10^23 atoms of C weighs 12g
Hence,1 atom weighs 12/(6.023×10^23) g
And 1/12th of this is simply 1/(6.023×10^23) which when multiplied by (6.023×10^23) gives 1 as the product.
Hope it helps! :)
vineet_kothari 88 days ago