Question

Caffeine has a molecular weight of 194. If it contains 28.9 % by mass of nitrogen, the number of atoms of nitrogen in one molecule of caffeine is:
(A) 4
(B) 6
(C) 2
(D) 3

Answer 1

First of all calculate the mass of nitrogen present in 1 mole of caffeine using the mass percentage formula. As we know that mass percentage is calculated by taking the mass of the particular component divided by the mass of the entire mixture multiplied by 100. Then find the number of moles of nitrogen atom present in 1 mole of caffeine. This will be equal to the number of nitrogen atoms present in 1 molecule of caffeine.
Formula used:
-Mass percentage:
$\% mass = \dfrac{{wt.component}}{{wt.mixture}} \times 100$ (1)

Complete answer:
-According to the question the molecular weight of caffeine is 194 gram per mole. So, we can say that 194 grams are present in one mole of caffeine.
-The question also says that caffeine has 28.9% mass of nitrogen in it.
Mass percentage is basically an expression of concentration and describes the component or compound in a particular mixture. Mass percentage is calculated as the mass of a component divided by the total mass of the mixture, multiplied by 100.
Mathematically it can be written as:
$\% mass = \dfrac{{wt.component}}{{wt.mixture}} \times 100$ (1)
Let the weight of nitrogen present in it be W. By using equation (1) and putting the values in it:
$28.9 = \dfrac{W}{{194}} \times 100$
W = $\dfrac{{28.9 \times 194}}{{100}}$
= 56.066 g/mole
Weight of nitrogen present in 1 mole of caffeine is 56.066 g/mole.
-We will now calculate the moles of nitrogen associated with the obtained weight.
$Moles = \dfrac{W}{M}$
Where, W = given weight = 56.066 g/mol; and M = molecular weight or atomic weight = 14 g
Moles = $\dfrac{{56.066}}{{14}}$
= 4.0047 ≈ 4
So, we can say that 1 mole of caffeine contains 4 moles of nitrogen atoms; which means that 1 molecule of caffeine will contain 4 atoms of nitrogen.

Hence, the correct option will be: (A) 4.

Note:
We should know that caffeine is a natural stimulant most commonly found in tea, coffee, and cacao plants. It works by stimulating the brain and central nervous system, helping you stay alert and prevent the onset of tiredness. If we consume caffeine, it gets quickly absorbed from the gut into the bloodstream. From there, it travels to the liver and is broken down into compounds that can affect the function of various organs. It mainly affects our brain.
It functions by blocking the effects of adenosine, which is a neurotransmitter that relaxes the brain and makes us feel tired. We should know that adenosine levels build up over the day, making us increasingly more tired and causing us to want to go to sleep. Caffeine helps us stay awake by connecting to adenosine receptors in the brain without activating them. This blocks the effects of adenosine, leading to reduced tiredness. It may also increase blood adrenaline levels and increase brain activity of the neurotransmitters dopamine and norepinephrine.