The Avogadro constant is {6.0\times 10^{23} mol^{-1}}.
How many hydrogen atoms are there in 0.420 g of cyclohexane?
[Ar: H = 1; C = 12]
A. 1.8\times 10^{23}
B. 1.8\times 10^{22}
C. 3.0\times 10^{21}
D. 3.0\times 10^{22}
E. 3.6\times 10^{22}
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The general formula for cyclic Alkanes is C_nH_{2n}
Meaning the formula for cyclohexane is C_6H_{12}
The first step we can take is finding the M_r of cyclohexane.
If the A_r of H=1 and that of C= 12, then (12x6)+(1x12)= 84
If one mole of cyclohexane would give us 84 grams of product, then how many moles would 0.420 grams give us?
1 = 84
x= 0.420
84x= 0.420
x= 0.005 moles
If one mole of cyclohexane contains 6.0 \times 10^{23} mol^{-1} molecules, then how many molecules are there in 0.005 moles?
1 =6.0 \times 10^{23} mol^{-1}
0.005 = x
x= 3\times 10^{21} total molecules of cyclohxane
We now can tell that the number of hydrogen atoms is 12x that of cyclohexane molecules, since there would be 12 hydrogen atoms in a cyclohexane molecule. That would help us compute that the number of hydrogen atoms are:
12\times (3\times 10^{21})= 3.6\times 10^{22}
Our answer is E.
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The easier solution is to calculate how many grams of hydrogen we have in 0.42 g of cyclohexane.
We know that in 84 g of cyclohexane there is 12 g hydrogen.
12× 0.42 ÷ 84 = 0.06 hydrogen g
Now, we know that 1 mol of hydrogen has 6×10²³ atoms.
So the number of atoms in 0.06 g hydrogen would be:
6 ×10²³ × 0.06 = 36 × 10²¹
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where did you get the 0.84 from?
Cuz cyclohexane has 12 hydrogen and 6 carbon, so :
(12×1)+(6×12)= 84