Question

Assuming that a cell contains a 310 mOsm solution of nonpermeable solutes, some molecular weighs are as follows:

H=1, O=16, S-32, Ba=137, C=12, Na=23, Cl=35

1. What're the molarities of a 10% sucrose solution and a .90% sodium chloride solution?

2. What would be the fate of an erythrocyte placed into a 10.00% sucrose solution? Explain what happens. (Remember what happens to erythrocytes when placed into different solutions)

Answer 1

#1.A. Molarity of 10% sucrose

10% sucrose solution means there is 10.0 g sucrose in 100.0 mL of its solution.

So,

            Volume of solution = 100.0 mL = 0.100 L

Mass of sucrose = 10.0 g

Now,

            Moles of sucrose = Mass of sucrose / Molar mass

                                                = 10.0 g / (342.0 g/ mol)

                                                = 0.02924 mol

Molarity of sucrose = Moles of sucrose / Volume of solution in liters

                                    = 0.02924 mol / 0.100 L

                                    = 0.2924 mol/ L                                            ; [1 M = 1 mol/ L]

                                    = 0.2924 M

#1.B. Molarity of 0.9 % NaCl

            Volume of solution = 100.0 mL = 0.100 L

Mass of NaCl = 0.9 g

Now,

            Moles of NaCl = 0.9 g / (58.0 g/ mol) = 0.0155 mol

Molarity of NaCl = Moles of NaCl / Volume of solution in liters

                                    = 0.0155 mol / 0.100 L

                                    = 0.155 M

#2. Osmolarity is the number of osmoles of solute per liter of solution.

Osmole is the number of moles of a solute that contributes to osmolarity of the solution.

For an electrolyte (produces two or more ions in aqueous solution), number of osmoles is equal to number of ions produced per mol electrolyte.

For non-electrolytes like sucrose, av’t Hoff factor, i=1.

So,

Osmolarity = (i x Molarity) osmole ;     [where, i = Van’t Hoff factor]

Now,

            Osmolarity of 10% sucrose = 1 x molarity of sucrose solution

                                                            = 0.2924 osmol

                                                            = 292.4 mosmol                 ; [1 mosmol = 10^-3 osmol]

# Given, osmolarity of erythrocytes = 300 mosmol

            Osmolarity of sucrose solution = 292.4 mosmol

When in contact, water will move from the solution of lower osmolarity to that of higher osmolarity.

So, when erythrocytes are placed in 10% sucrose solution, water will move into RBCs from sucrose solution until equilibrium is established.

Note: The calculated osmolarity of sucrose is nearly equal to that of RBCs, but NOT exactly equal. It’s slightly lower than 300 mosmol.