Assuming 65 % of a human body is water (H2O) and the rest is mostly carbon (C), estimate the number of atoms in a human body

Hi, So we can say 65 g is water which is equaivalent to 3.6 moles of water. 35 g of carbon which is equivalent to 2.9 moles of carbon. So in total there 6.5 moles in total which is equivalent to 6.5X6.023X 10^23so the answer is = 3.94 X 10^25 atoms 
Mahima L.
12 July 2016
Hi Gemma, an average human adult has a mass of around 70-80kg so lets estimate this as 75kg,65% of this is water so we have 75kgX65%=48.75kg of water, we also have 35% carbon so there is 75kgX35%=26.25kg of carbon.we can then convert these masses to number of moles using mass/molar mass=number of moles (M/n=Mol), in water the molar mass is that of 2 hydrogen atoms plus 1 oxygen atom(found from the periodic table)=1+1+16=18 and the molar mass of carbon(also form the periodic table)=12.This means we have for water 48750g/18 = 2708 molesand for carbon 26250g/12 = 2188 moles, this equals 4896 moles in total in an average humanthere are 6.022x10^23 atoms in a mole (Avagadro's constant) and so this means there are 4896x(6.022x10^23) = 2.95x10^27 atoms in an average human bodyso to estimate it is about 3x10^27 atoms :)
12 July 2016
Dear GemmaLet's consider average mass of an adult human = 70 KgTotal mass of carbon = 35x70÷100                                       = 24.5 KgMoles of Carbon atoms = 24.5x1000÷12 molSo , Number of Carbon atoms in a human body = 24.5x1000x 6.022 x 10^23 ÷ 12= 1.229 x 10 ^27 atoms
28 February 2019
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