## blahhhhh12345678 one year ago How many molecules of H2O are there in 1.0 g of H2O?

1. dimensionx

in 1.0 grams of H2O. okay so.. You are given grams (mass). To go from mass to molecules, you have to convert: Mass -> Moles -> Molecules. To go from mass to moles, Divide by the molecular weight. To go from moles to molecules, multiply by 6.02 x 10^23.

2. abb0t

convert to moles. then use avogradro's number $$6.022 \times 10^{23}\frac{ atoms }{ mol }$$

3. blahhhhh12345678

I do not know how to do this ..

4. dimensionx

Sure you do ;) Let's take it step by step. You are given 1 gram of H2O, and need to divide this by the molecular weight. The molecular weight can be found by adding up all the individual masses of the atoms. How many atoms of Hydrogen are in H2O? How many atoms of Oxygen are in H2O? Multiply the number of atoms of Hydrogen by the atomic mass of hydrogen. Multiply the number of atoms of Oxygen by the atomic mass of oxygen. Add these two values together, what do you get?

5. blahhhhh12345678

2 atoms in Hydrogen and 1 atom of Oxygen. atomic mass of hydrogen is 1 2*1=1 atomic mass of oxygen is 15.9994 15.994*1=15.9994 Added together is 16.9994

6. Mertsj

Do you have it now? Or do you still need help?

7. blahhhhh12345678

I don't know if 16.9994 is the answer?

8. Mertsj

Hang on. I'll figure it out

9. blahhhhh12345678

Okay thank you

10. Mertsj

No. Obviously no. It's way too small.

11. blahhhhh12345678

I have no clue what I am doing

12. Mertsj

Do you know what a mole is?

13. blahhhhh12345678

Not really

14. Mertsj

That might be your trouble. A mole of any compound or element is the molecular weight in grams. So if you had a scale and wanted to weigh out a mole of water, you would weigh out 18 g because 2H=2, O = 16 and the molecular weight is 18

15. Mertsj

Do you understand that?

16. blahhhhh12345678

No, I am so sorry

17. Mertsj

Do you know how to find molecular weight?

18. blahhhhh12345678

I looked it up online

19. Mertsj

So I take that to be a yes?

20. Mertsj

Do you have a periodic table?

21. blahhhhh12345678

I looked up the weight online. and I do, yes

22. Mertsj

Find the molecular weight of HCl

23. blahhhhh12345678

36.46094

24. Mertsj

So a mole of HCl is 36.46094 grams.

25. Mertsj

Now here is the next important point: a mole of "stuff" has 6.023 x 10^23 molecules in it.

26. Mertsj

That's Avagadro's number. That is something you cannot reason out you have to learn it.

27. Mertsj

So lets say I have some carbon. A mole of carbon is 12 grams. I have 24 grams. How many moles is that?

28. blahhhhh12345678

12

29. blahhhhh12345678

2

30. Mertsj

Yes. Good. Now remember each mole is 6.023 x 10^23 molecules so how many molecules of carbon do I have?

31. blahhhhh12345678

I suppose I do not understand how 6.023 X 10^23 works because if there are 2 moles how exactly do I work that in?

32. dimensionx

If you have 1 mole of "stuff", you have exactly 6.023 x 10^23 molecules. If you have 2 moles of stuff, you would multiply Avogadro's number by 2.

33. Mertsj

Would you agree that 2 moles is twice as much as 1 mole and so I should multiply by 2?

34. dimensionx

^

35. Mertsj

And $2(6.023 \times 10^{23)}=12.046 \times 10^{23}$

36. Mertsj

So that is the number of molecules in 2 moles of carbon.

37. Mertsj

38. blahhhhh12345678

so we aren't really solving the entire equation because when I did that I had a ginormous number

39. dimensionx

@Mertsj sorry didn't mean to tread on you xD blahhhh, We are taking it step-by-step ;)

40. dimensionx

So we know that we have 2 Hydrogens and 1 Oxygen. 2 x Hydrogen 1 x Oxygen 2 x (1) = 2 1 x (16) = 16 Total Molecular Weight = 18

41. dimensionx

Do you understand how I got this value?

42. blahhhhh12345678

yes.

43. Mertsj

So why isn't this problem done yet?

44. blahhhhh12345678

because I do know how to apply this... I am thinking at 18 is the answer. I am sorry It is just not clicking.

45. Mertsj

18 grams is 1 mole of water. How many moles is 9 grams of water?

46. blahhhhh12345678

.5

47. Mertsj

That would be 9/18 right?

48. blahhhhh12345678

yes

49. Mertsj

How many moles is 3 grams of water?

50. Mertsj

Just tell me the fraction.

51. blahhhhh12345678

50 g

52. Mertsj

3/18

53. Mertsj

I think I see a pattern here: I will draw it because I want you to see it too.

54. blahhhhh12345678

okay!

55. Mertsj

|dw:1364947152844:dw|

56. Mertsj

Do you see the pattern?

57. Mertsj

How many moles is 1 g water?

58. blahhhhh12345678

1/18

59. Mertsj

Very good.

60. Mertsj

Now we know we have 1/18 mole of water and each mole has 6.023 x 10^23 molecules. So we should multiply 1/18 by 6.023 x 10^23 to find how many molecules we have.

61. Mertsj

What do you get?

62. blahhhhh12345678

hang on

63. blahhhhh12345678

my calculator says 0

64. Mertsj

|dw:1364947644414:dw|

65. blahhhhh12345678

I am not getting those numbers..

66. Mertsj

Type 6.023 divided by 18 equals

67. Mertsj

Did you get .334611111111 ??

68. blahhhhh12345678

yes.

69. Mertsj

That has to be multiplied by 10^23 but that is WAAAAAAY to many zeros so we we just write it: $.33461111 \times 10^{23}$

70. Mertsj

However we have two problems. 1. That is too many significant digits 2. It is not in scientific notation.

71. Mertsj

So we will round it to .33 So now we have: $.33 \times 10^{23}$

72. Mertsj

But that is still not scientific notation so we will change the .33 to 3.3 but that means we have to change the exponent to 22: $3.3 \times 10^{22}$

73. blahhhhh12345678

Oh! that mas sense

74. blahhhhh12345678

makes*

75. Mertsj

Excellent. Now. You can do these problems without really understanding them if you know or can find the right conversion factors and just set it up so that the units turn out right. Do you care to see that?

76. blahhhhh12345678

Of course!

77. Mertsj

78. Mertsj

$1 gram water \times \frac{1 mole water}{18g water} \times \frac{6.023 x 10^{23}molecules}{1 mole water}$

79. Mertsj

Notice how the units cancel just like numbers and you end up with the number of molecules which is just what the problem asked for.

80. Mertsj

So if the units come out right, you know you have the numbers right.

81. blahhhhh12345678

so that would translate to 1 x 18/18= 1 x 6.023x10^23/18?

82. Mertsj

yes. multiply all the numerators and divide by all the denominators.

83. Mertsj

But the powers of 10...just leave that til the end.

84. blahhhhh12345678

when I did that I got a massive number..since I already knew it 3.3x10^22 it made sense how do I figure it out when there is no decimal?

85. Mertsj

The exponent 22 means move the decimal point 22 places to the right. That's why we use scientific notation. Who wants to mess with all those zeros?

86. blahhhhh12345678

right, it is like I get it for a second then it all goes away. when I did it using the formula you gave canceling everything out I just got a massive number 334111111111...

87. blahhhhh12345678

33461*

88. Mertsj

Exactly. You can enter it into your calculator if you use the EE button to enter the exponent. At least that's the way it is on my calculator. I enter 6.023 EE23 and then it manages the exponents and scientific thing for me.

89. blahhhhh12345678

Oh I get it! .... so when it is a .xy number you move the decimal and it becomes x.y^22 instead of ^23. and if it turns out a x.y number it stays ^23?

90. Mertsj

Bingo!!

91. blahhhhh12345678

OH MY GOD YOU ARE POSITIVELY AMAZING AND YOU HAVE NO IDEA HOW MUCH I APPRECIATE YOUR TIME AND PATIENCE! In the future if I come across a problem like this I would like to run it by you so you check it? Or any other problems that threaten a brain aneurysm lol

92. Mertsj

Glad to be of service. Remember Jesus said, "Whoever would be great among you, let him be your servant."

93. blahhhhh12345678

Beautiful. I wouldn't want to call you a servant but, I love to learn from those who know more than I do.

94. Mertsj

Me too.