中三數學!!!!!幫幫手!!!!PLEASE!!!!

2007-09-18 3:41 am

1. (4.7x10^5)^2+(4.7x10^5)(5.3x10^5)

2. 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^-27 kg and 2.67x10^-26 kg respectively.(For part b, express the answers in scientific notation. Give the answer correct to 3 significant figures if necessary).
a What is the percentage of the weight of hydrogen atoms in a water molecule?
b How many water molecules are there in 300g of pure water?

回答 (1)

用戶84916

2007-09-18 3:53 am

✔ 最佳答案

1. (4.7x10^5)^2+(4.7x10^5)(5.3x10^5)
=4.7^2x10^10 + 4.7x5.3x10^10
=4.7x10^10 (4.7 + 5.3)
=4.7x10^11

2. (a) % of weight of hydrogen atoms
= 2x1.67x10^-27/(2x1.67x10^-27+2.67x10^-26) x100%
= 2x1.67/(2x1.67 + 26.7) x 100%
= 11.1%
(b) A water molecule weighs
2x1.67x10^-27+2.67x10^-26
= 3.004 x 10^-26

No. of water molecules in 300g of pure water
= 300/(3.004 x 10^-26) x 2
= 2.00 x 10^28 (corrected to 3 sig. fig.)

2007-09-17 19:55:13 補充：
Correction:No. of water molecules in 300g of pure water= 300/(3.004 x 10^-26) = 1.00 x 10^28 (corrected to 3 sig. fig.)

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