Question

What mass of salt would you need to add to 1.00kg of water to achieve a freezing point of -5 degrees C

Answer 1

To achieve a freezing point of -5 °C, you need to add approximately 78.55 grams of salt to 1.00 kg of water, using the concepts of freezing point depression and the properties of NaCl when dissolved in water.

Step-by-step explanation:

To calculate the mass of salt required to lower the freezing point of water to -5 degrees Celsius, we need to use the concept of freezing point depression.

The freezing point depression (ΔTf) is given by the formula ΔTf = i · Kf · m, where 'i' is the van't Hoff factor (which is 2 for NaCl, since it dissociates into two ions, Na+ and Cl-), 'Kf' is the freezing point depression constant for water (1.86 °C-kg/mol), and 'm' is the molality of the solution.

The problem gives us the desired freezing point depression as 5 °C (since water normally freezes at 0 °C). We can start by rearranging our formula to solve for 'm': m = ΔTf / (i · Kf).

Plugging in the values, we get m = 5 °C / (2 · 1.86 °C-kg/mol) = 1.344 mol/kg. Since the question specifies 1.00 kg of water, we need 1.344 moles of NaCl. The molar mass of NaCl is approximately 58.44 g/mol, so the mass of NaCl needed is 1.344 mol · 58.44 g/mol = 78.55 g.

Therefore, to achieve a freezing point of -5 °C, you need to add approximately 78.55 grams of salt to 1.00 kg of water.

Answer 2

The type of salt to be added to the water is not known from the question but no worries, I will try to give you the step by step procedure to answer any type of question similar to this.

To answer this question, we should know some facts.

1. the molar freezing point depression constant of water (Kf) = 1.86 K kg/mol

2. the molar mass of the salt if NaCl = 58.5 g/mol ; KCl = 74.5 g/mol

3. since the salt can dissociate if NaCl or KCl into two ions, the Van't Hoff factor ( i )= 2

Note that: the change in freezing point, molarity, deepression constant and van't Hoff factor are related by this formula;

ΔTf = i Kf m

So lets take NaCl as the salt:

Molar mass = 58.5 g/mol

Van't Hoff factor = 2

1. calculate the number of moles

So we can calculate the molarity of the salt NaCl from the formula;

m = ΔTf / i Kf

m = 5 / 2 * 1.86

m = 5 / 3.72

m = 1.344 mol/kg

2. calculate the number of moles of the salt required

Next is to multiply the molarity by the mass of water. Density of water = 1kg/L

number of moles = 1.344 mol/kg * 1 Kg/L * 1 kg water

number of moles = 1.344 moles.

3. calculate the mass of the salt.

numner of moles = mass / molar mass

mass = number of moles * molar mass

mass = 1.344 * 58.5

mass = 78.624 g of NaCl salt.

You can follow these steps to solve for the type of salt you are given in the question.