Question

Molality Problems (Need ASAP!) 100 POINTS!!! Thanks :)

1. What is the molality of a solution in which 0.135 moles of CO2 is dissolved in 490mL of water? (1kg of water = 1L of water)

2. What is the molality of a solution in which 23g of NaCl are dissolved in 250g of water?

3. How many moles of HC1 are needed to prepare a 3m (3molal) solution with 1.5Kg of water?

Answer 1

1.  0.2755 M

2. 1.574 M

3.   4.5 moles

Explanation:

Molality is defined as the number of moles of  solute dissolved in a kg of solvent;

1)

If 0.135 moles of CO2 is dissolved in 490mL of water, how much moles are in 1000 mL (1 kg) of water;

0.135 : 490

x : 1000

490x = 1000 * 0.135

x = 135 /490

x = 0.2755 moles

= 0.2755 M

2)

If 23g of NaCl are dissolved in 250g of water, how much NaCl in dissolve in 1000g  (1kg) of water;

23 : 250

x : 1000

250x = 1000 * 23

x = 23000/250

x = 92 g

To find many moles of NaCl are in 92g (Remember molar mass of NaCl is 58.44 g/mol) ;

92/ 58.44

= 1.574 moles

= 1.574 M

3)

A 3 M HCl solution means there are 3 moles of HCl in 1L (1 kg) water.

3 : 1

x : 1.5

x = 1.5 * 3

x = 4.5

= 4.5 moles