Question

Caffeine is a compound found in some natural coffees and teas and in some colas. a. Determine the empirical formula for caffeine, using the following composition of a 100.00-g sample. 49.47 grams of carbon, 28.85 grams of nitrogen, 16.48 grams of oxygen, and 5.20 grams of hydrogen b. If the molar mass of caffeine is 194.19 g/mol, calculate its molecular formula.

Answer 1

Answer: The molecular formula will be
`C_8N_4H_(10)O_2`

Step-by-step explanation:

Mass of C= 49.47 g

Mass of N = 28.85 g

Mass of O = 16.48 g

Mass of H = 5.20 g

Step 1 : convert given masses into moles.

Moles of C =
`\frac{\text{ given mass of C}}{\text{ molar mass of C}}= (49.47g)/(12g/mole)=4.12moles`

Moles of N =
`\frac{\text{ given mass of N}}{\text{ molar mass of N}}= (28.85g)/(14g/mole)=2.06moles`

Moles of O =
`\frac{\text{ given mass of O}}{\text{ molar mass of O}}= (16.48g)/(16g/mole)=1.03moles`

Moles of H =
`\frac{\text{ given mass of H}}{\text{ molar mass of H}}= (5.20g)/(1g/mole)=5.20moles`

Step 2 : For the mole ratio, divide each value of moles by the smallest number of moles calculated.

For C =
`(4.12)/(1.03)=4`

For N =
`(2.06)/(1.03)=2`

For O =
`(1.03)/(1.03)=1`

For H =
`(5.20)/(1.03)=5`

The ratio of C : N: O: H = 4: 2: 1: 5

Hence the empirical formula is
`C_4N_2OH_5`

The empirical weight of
`C_4N_2OH_5` = 4(12)+2(14)+1(16)+5(1)= 97 g.

The molecular weight = 194.19 g/mole

Now we have to calculate the molecular formula.

`n=\frac{\text{Molecular weight}}{\text{Equivalent weight}}=(194.19)/(97)=2`

The molecular formula will be =
`2* C_4N_2H_5O=C_8N_4H_(10)O_2`