How do you solve these molar problems?

Question

Please explain thoroughly how to solve these . Thank you !
1.) Sucrose has a molecular mass of 342 daltons. To make a 2-molar (2 M) solution of sucrose, _____.

a) stir 342 g of sucrose in water to dissolve the sugar, and then add enough water to bring the total volume of the solution to 1 L
b) stir 684 g of sucrose in water to dissolve the sugar, and then add enough water to bring the total volume of the solution up to 2 L
c) stir 684 g of sucrose in water to dissolve the sugar, and then add enough water to bring the total volume of the solution up to 0.5 L
d)stir 342 g of sucrose in water to dissolve the sugar, and then add enough water to bring the total volume of the solution up to 2 L
e) stir 342 g of sucrose in water to dissolve the sugar, and then add enough water to bring the total volume of the solution up to 0.5 L

2.) A mole of ethyl alcohol weighs 46 g. How many grams of ethyl alcohol are needed to produce 1 L of a 2-millimolar (2 mM) solution?

a) 92 g
b) 9.2 g
c) 0.92 g
d) 0.092 g
e) 0.0092 g

3.) How do you make 0.5 molar solution of sodium chloride (NaCl)?


The atomic mass of Na is 23 daltons and that of Cl is 35.5.

4.)How many grams of acetic acid (C2H4O2) would you use to make 10L of a 0.1M aqueous solution of acetic acid?


(Note: The atomic masses, in daltons, are approximately 12 for C, 1 for H, and 16 for O)

Answer 1

M stands for moles/liter. 

 

1) The atomic mass of sucrose is 342 daltons. This means the mass of one mole of sucrose is 342 grams. To make a 2 molar solution of sucrose, you must have 2 moles of sucrose in 1 liter of solution. 2 moles of sucrose would have a mass of 684 grams. This would be in a solution with a volume of 1 liter. Only one of these choice would give us 684 grams of sucrose in 1 L of solution. E

 

2. 1 Mole = 46 grams

 

    1 Milli Mole = .046 grams

    2 Milli Moles = 2 * .046 grams = .092 grams

 

The answer is d.

 

3. 1 mole of salt has a mass of 58.5 grams. 1/2 mole of salt would have a mass of 29.25 grams then. To make a 0.5 molar solution of salt, you should take 29.25 grams of salt & mix it with water until the total volume of the solution is 1 liter. 

 

4. A 10 liter aqueous solution of 0.1M acetic acid would have 1 mole of acetic acid per 10 liters of solution. 1 mole of acetic acid would have a mass of:

 

12 grams *2 + 1 gram *4 + 16 grams *2 = 24 grams + 4 grams + 32 grams = 60 grams