Question

A cup of coffee contains 500 mg of caffeine which has the chemical formula C8H10N4O2. Which one of the following is the number of nitrogen atoms present in this amount of caffeine?

A 1.55x10^21
B 6.21x10^21
C 1.55x10^23
D 6.21x10^24

the answer is B but i dont know the method!

Answer 1

= 6.21x10^21

Step-by-step explanation:

It's the train track method, where you try to cancel all the units and end with units that you need. Here we want to end with number of atoms of N.

500 mg N x 1 g N/1000 mg N x 1 mol N/194.19 g N x 4 mol N/1 mol of caffeine x (6.022 x 10^23 particles of N / 1 mol N)

= 6.21x10^21