Table of Contents

• Prequiz...
• 1: What is a mole?
• 2: How much does a mole weigh?
• 3: Molar weights made easy
• 4: ...but not quite that easy
• 5: Using the Periodic Table
• 6: Water
• 7: Practice with sulfuric acid
• 8: ...and caffeine
• 10: ...and rubies
• 11: Going backwards
• 11: Summary

Heading the other direction

Let's say I tell you that a sugar cube weighs 5 grams. Given the formula C₆H₁₂O₆, how many mols would that be?

If this kind of problem is unfamiliar to you, start by thinking about what you do know how to do. For example, you do know how to calculate the molar weight of sugar. So, given the formula above, what's the molecular weight of sugar?

Calculate it on your own, then click here to check:

1 H 1.01

2 He 4.00

3 Li 6.94

4 Be 9.01

5 B 10.81

6 C 12.01

7 N 14.01

8 O 16.00

9 F 19.00

10 Ne 20.18

11 Na 22.99

12 Mg 24.30

13 Al 26.98

14 Si 28.09

15 P 30.97

16 S 32.07

17 Cl 35.47

18 Ar 39.98

19 K 39.10

20 Ca 40.09

21 Sc 44.96

22 Ti 47.88

23 V 50.94

24 Cr 51.94

25 Mn 54.94

26 Fe 55.85

27 Co 58.93

28 Ni 58.69

29 Cu 63.55

30 Zn 65.39

31 Ga 69.72

32 Ge 72.61

33 As 74.92

34 Se 78.96

35 Br 79.90

36 Kr 83.80

 

So the molar weight of sugar is 180 grams: in other words

180 grams / 1 mol.

But what we want to know is how many moles per gram, not grams per mole ... therefore we need to flip the fraction "upside down":

1 mol / 180 grams = 0.0055 mol/g

and we have 5 grams, so that's

1 cube = 5 g * 0.0055 mol/g= 0.028 moles.