You have 500 ml of a 4.00 m solution of kno3 from a previous experiment, but now you need 250 ml of a 0.500 m concentration of the same solution. how much of the stock solution will you need to make the dilute solution?

9 ml

20.0 ml

23.08 ml

9.09 ml

31 ml

what is the freezing point of a solution of 498ml of water (solute) dissolved in 2.50 l of ethanol (solvent), c2h5oh?
the density of c2h5oh is 0.789g/cm3. (remember that water has a density of 1.0 g/cm3.)
-25.49°c

-142°c

-89.0°c

-73.3°c

-43.3°c

First question: 31 mL

Second question:  T° freezing solution = -148.7°C

(It is logical to say -142°C if we look at the choices, but the value does not match)

Explanation:

First question

Let's use dilution factor to solve the first question:

Diluited volume / Concentrated volume = M concentrated / M diluited

250 mL / Concentrated volume = 4 M / 0.5M

250 mL / Concentrated volume = 8

250 mL / 8 = Concentrated volume ⇒ 31 mL

We can also apply this formula

C₁ . V₁ = C₂ . V₂

4M . V₁ = 0.5M . 250 mL

V₁ = 31.25 mL

Second question → Colligative property about freezing point depression

ΔT frezzing = Kf . m . i

In this case i, is the Van't Hoff factor and it values 2

H₂O → H⁺  +  OH⁻

m = molal (mol of solute in 1kg of solvent)

We have the volume of solvent, so with density we can know its mass.

Solvent mass = solvent volume / solvent density

Solvent mass = 2500 mL / 0.789 g/mL → 3168.5 g

(be careful with the units)

Solute mass = solute volume / solute density

498 g → solute mass, as the volume is 498 mL and density is 1 g/mL

Moles of water (mass / molar mass) ⇒ 498 g / 18 g/m = 27.6 moles

3168.5 g = 3.1685 kg

27.6 m / 3.1685 kg = 8.73 mol/kg

ΔT frezzing = T° freezing pure solvent - T° freezing solution

-114°C - (-T° freezing solution) = 1.99 °C/m . 8.73 m . 2

- T° freezing solution = 1.99 °C/m . 8.73 m . 2 + 114°C

T° freezing solution = -148.7°C

B. The molecules move more slowly and their average kinetic energy decreases.

Explanation: Kinetic energy is the energy possessed by a body by virtue of its motion.

Average kinetic energy is defined as the average of the kinetic energies of all the particles present in a system. It is determined by the equation:

R = gas constant

T = temperature of system

From above, it is visible that kinetic energy is directly related to the temperature of the system. So when the temperature is decreased from to ,the average kinetic energy of the system will also decrease and the molecules will start moving more slowly.