Question

Aspirin has the formula C9H8O4. How many atoms of oxygen are there in a tablet weighing 360mg?

Answer 1

The relative molecular mass of C9H8O4:  = 12x9+1x8+16x4= 180
                                                                 C       H      O

Ratio of C:H:O = 108:8:64      =      27:2:16    total ratio is 45

The mass of oxygen is 16/45 ----> 16/45 x 360 mg = 128 mg

Moles of oxygen =   mass/molar mass    =     128/16  = 8 moles

Avagadro's law:  In 1 mole of a substance, there is 6.023 x 10^23 atoms.

Therefore in 8 moles there will be, (8 x 6.023 x 10^23)

=  4.8184 x 10^24 atoms
 

Answer 2

Explanation:

• Given weight of Aspirin tablet: 360 mg.......in grams.....360 × 10-³ g
• molar mass of C9H8O4 = 9×12+1×8+16×4 = 108+8+64 = 180 g/mole
• moles = 360 × 10-³ ÷180 = 2 × 10-³ mole
• number of molecules in one mole = 6.023 × 10^23 ( Na)
• therefore, number of molecules in 2 × 10^-3 × Na
• 1 molecule of aspirin contains 4 atoms of oxygen

so  2× 10^ -3  × Na molecules will contain = 4 ×2 ×10^-3 × Na

                                                                     =  8 × 10 ^-3 × 6.022 ×10^23

                                                                      =  48.184 × 10^20

                                                                       = 4.8184 × 10^21 oxygen atom.