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# 中3~Laws of Indices[要列式]

A water molecule consists of two hydrogen atoms and one oxygen atom. The weights of a hydrogen atom and an oxygen atom are 1.67x10^-27kg and 2.67x10^-26kg respectively.

(For a & c, express the answers in scientific notation. Give the answers cor. to 3 sig. fig. if necessary.)

a,Find the weight of a water molecule.[3.004x10^-26]

b,What is the percentage of the weight of hydrogen atoms in a water molecule?[11.1%]

c,How many water molecules are there in 300g of pure water?[9.99x10^24]

### 1 Answer

- Audrey HepburnLv 71 decade agoFavorite Answer
a. Weight of a water molecule

= [(1.67 X 10-27) X 2] + (2.67 X 10-26)

= 0.334 X 10-26 + 2.67 X 10-26

= 3.004 X 10-26 kg

b. Weight of hydrogen atoms in a water molecule

= (1.67 X 10-27) X 2

= 3.34 X 10-27 kg

So, percentage of weight

= (3.34 X 10-27) / (3.004 X 10-26) X 100%

= 3.34 / 3.004 X 10-27 – (-26) X 100%

= 1.11 x 10-1 X 100%

= 11.1 % (cor. to 3 sig. fig.)

c. 300 g = 0.3 kg

So, no. of water molecules

= 0.3 / (3.004 X 10-26)

= 0.3 / 3.004 X 10-(-26)

= 0.0999 X 1026

= 9.99 X 1024 (cor. to 3 sig. fig.)

Source(s): Myself~~~