If the total number of atoms in a sample of potassium dichromate is $2.13\cdot10^{24}$ , then the amount of chromium in this sample is? the answer that I've come up with is $367.64\ \mathrm g$ but in fact the solution in my book is $33.4\ \mathrm g$.

  • $\begingroup$ Well how did you come up with your answer? $\endgroup$ – bon Feb 17 '16 at 18:55
  • $\begingroup$ 2.13*10^24/6.022*10^23=3.54 mol *2=7.07 since the molecular form of the compound is K2Cr2O7 , then I"v calculated the mass by multiplying moles by 52 $\endgroup$ – 119 j7fly Feb 17 '16 at 19:01

I like to solve such assignments using Python as a scientific calculator.

import scipy.constants 

# number of atoms in sample
N_ATOMS_TOTAL = 2.13E+24 

# atomic mass of chromium
M_CR = 51.9961 # g/mol

# ratio of chromium atoms to total atoms in K2Cr2O7
CR_RATIO = 2/11

# moles of Cr in sample
n_cr = N_ATOMS_TOTAL * CR_RATIO / scipy.constants.Avogadro

mass_cr = n_cr * M_CR # mass of chromium in g


Let me explain:

  • You have an unknown mass (in gramm) of potassium dichromate, $\ce{K2Cr2O7}$.

  • The total number of atoms in this sample is $2.13\cdot10^{24}$.

How can you calculate the number of chromium atoms in the sample?

  • You need to determine the ratio of $\ce{Cr}$ to all atoms in $\ce{K2Cr2O7}$, and that's $\frac{2}{11}$ atoms.

I know the number of $\ce{Cr}$ atoms now, but how many moles is that?

  • You know that 1 mol of an element has $6.02214129\cdot10^{23}$ atoms of that element (Avogadro's number).
  • Divide the number of $\ce{Cr}$ atoms determined before by Avogadro's number to determine the number of moles of $\ce{Cr}$.

I know the number of moles of $\ce{Cr}$ now, but what is the weight?

  • You know that the standard atomic weight of $\ce{Cr}$ is 51.9961.
  • Multiply the standard atomic weight with the number of moles to get the weight.
  • $\begingroup$ Im so confused :( , I know that when a substance like H20 has 2.5 mol it means H has number of moles=2*2.5 . but If I used the way u solved the question it will be 2 H atoms+1 O atom=3 atoms , to calculate H it will be (2/3)*2.5 and the result is different then :( $\endgroup$ – 119 j7fly Feb 17 '16 at 19:49
  • 1
    $\begingroup$ @119j7fly $\ce{K2Cr2O7}$ has 2 $\ce{K}$ + 2 $\ce{Cr}$ + 7 $\ce{O}$ atoms, that's 11 atoms in total. Two of these are $\ce{Cr}$ atoms. This means that the number of $\ce{Cr}$ atoms in the sample is 2/11 * the total number of atoms :) $\endgroup$ – Klaus-Dieter Warzecha Feb 17 '16 at 20:05
  • $\begingroup$ its very clear right now thanx a lot mr Klaus. $\endgroup$ – 119 j7fly Feb 17 '16 at 20:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.