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Begin by calculating the number of atoms in the Sun. Assume that the Sun is pure hydrogen with a density of $\pu{1.4 g cm^-3}.$ The radius of the Sun is $\pu{7E8 m},$ and the volume of a sphere is $V = \frac 4 3\pi r^3.$

This is what I did, since it worked with similar questions:

$$ N = \frac{\frac{4}{3}\pi(\pu{7E8})^3(1.4)(\pu{6.022E23})}{1.008} = \pu{1.2E51}, $$

but the answer is $\pu{1.2E57}.$ What have I done wrong?

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The problem is that you did not include units with your numbers. If you had, you would have easily seen that you are multiplying $\pu{m^3}$ and $\pu{cm^{−3}}$ and coming up with $1$ instead of $10^6.$

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