What is the concentration of sugar(molar mass=342g) in mol L-1 if its 20g are dissolved in 2L of water?
Answers
The molecular weights of each element are:
C: 12
H: 1
O: 16
So the molecular weight of the sugar is 12 x 12 + 22 x 1 + 11 x 16 = 342. So 20 grams is 20 / 342 = .058 moles. If the solution contains 2 liters, then the concentration is .058 /
The easiest way to answer this question is to first figure out the molar mass of the sugar in question. To do this multiply the number of individual atoms for a given element by its atomic mass.
12 X 12.01 g/mol = 144.12 g/mol C
22 X 1.008 g/mol = 22.176 g/mol H
11 X 16.00 g/mol = 176.00 g/mol O
Total = 342.296 ~ 342.30 g/mol C12H22O11
These numbers can be found using the periodic table
The second step is to take the number of grams of sugar you have; in this case 20g and divide it by the molar mass of the sugar 342.30 g/mol.
20g = 0.058 mol C12H22O11
342.30g/mol
The Third step is to take the number of moles and divide it by your total volume of 2L. Remember that for liquid concentrations, it is always in units of moles/L; this is also the definition of Molarity.
0.058 mol = 0.029 mol/L OR 0.029M C12H22O11
2L
I hope this clears up any confusion.