A mole of hydrogen atoms has 6.02 x 10^23 atoms. It occupies 22.414 L. How many hydrogen atoms are in 50.00 mL?

This is a simple case of ratios. (1 mol)/(22.4 L)=(n mol)/(.025 L) Then we cross multiply and we get 22.4n=.025 We divide each side by 22.4 to find n=.001116 mol Then to convert the moles to atoms we multiply, and cross-cancel the units (.001116 mol)/1 xx (6.02 xx 10^23 atms)/(1 mol) and we have 6.72 xx 10^20 atoms.  I've found the trick of cross-cancelling units to be a very effective mnemonic, it always makes sure you carry out the correct calculation to find the desired units.

Explanation:

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