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Is it true that the sea water is composed of about $86\%$ oxygen, $11\%$ hydrogen and $3\%$ of minerals? The chemical formula of water is $\ce{H2O}$ (two hydrogen and one oxgen) that shows that the number of hydrogen is greater than that of oxygen.

If the number of hydrogen is greater, then why does the sea water consist of $11\%$ hydrogen and $86\%$ oxygen, which is lesser than the oxygen?

The book which I am reading says which is confusing me:

... Seawater is composed of about $86\%$ oxygen, $11\%$ hydrogen and $3\%$ of minerals, consisting mainly of sodium and chlorine.

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The book that you're reading is measuring by mass.

If you have pure water then you would expect oxygen to make up $\frac{16}{16 + 2}\times 100\% \approx 89 \% $ by mass. Likewise, hydrogen would make up $\frac{2}{16 + 2}\times 100\% \approx 11 \% $ by mass.

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As it has already been mentioned, these are mass percentages, which are a bit imprecise. For the reference, CRC handbook of chemistry and physics [1, p. 14-17] lists elements' abundances in seawater near the surface in $\pu{mg L-1}$, alphabetically. I took 15 (starting with 16th element $\omega < 10^{-4}~\%$) most abundant elements from that array and calculated mass percentage for each:

\begin{array}{llr} \hline \text{Element} & \rho_i/\pu{mg L-1} & \omega/\% \\ \hline \ce{O} & \pu{8.57E5} & 85.8443 \\ \ce{H} & \pu{1.08E5} & 10.8182 \\ \ce{Cl} & \pu{1.94E4} & 1.9433 \\ \ce{Na} & \pu{1.08E4} & 1.0818 \\ \ce{Mg} & \pu{1.29E3} & 0.1292 \\ \ce{S} & \pu{9.05E2} & 0.0907 \\ \ce{Ca} & \pu{4.12E2} & 0.0413 \\ \ce{K} & \pu{3.99E2} & 0.0400 \\ \ce{Br} & \pu{6.73E1} & 0.0067 \\ \ce{C} & \pu{2.80E1} & 0.0028 \\ \ce{Sr} & \pu{7.9} & 0.0008 \\ \ce{B} & \pu{4.44} & 0.0004 \\ \ce{Si} & \pu{2.2} & 0.0002 \\ \ce{F} & \pu{1.3} & 0.0001 \\ \ce{N} & \pu{5.00E-1} & 0.0001 \\ \hline \end{array}

References

  1. Haynes, W. M.; Lide, D. R.; Bruno, T. J. CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data.; 2017; Vol. 97.
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  • $\begingroup$ You say the numbers are a bit imprecise, but they have to be, because not all seawater is the same! $\endgroup$ – Mr Lister Dec 18 '18 at 10:50

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