The best scientific estimates put the mass of the world's oceans at
about 1.37 x 10^21 kg:
Mass of the Oceans
The average salinity of the oceans is 35 parts per thousand:
...Throughout the world, the salinity of sea water averages about 35 o/oo
or 3 1/2% salt. Therefore, the oceans are 96.5% water, and amount of
water in the oceans is:
96.5% of 1.37 x 10^21 kg = 1.32 x 10^21 kg.
Each molecule of water is (by weight, and on average) 16 parts oxygen
and 2 parts hydrogen, so that water is:
2/16 = 1/8th = 12.5% hydrogen.
12.5% of 1.32 x 10^21 kg = 0.165 x 10^21 kg = 1.65 x 10^20 kg.
Therefore, there are 1.65 x 10^20 kg of hydrogen in the world's
oceans, which can also be expressed as 1.65 x 10^17 metric tons.
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