what is the molality of nitric acid in a concentrated solution of nitric acid (68% HNO3 by mass)?

Answer 1

Molality, m, is defined as moles of solute per kg solvent. For this problem we need to first find the moles of the solute and then the mass of the solvent.

1.) Moles of solute:

68% of HNO3 by mass means that for every 68 grams of HNO3 there are 32 grams of water, which is assumed to be solvent, unless otherwise stated. HNO3 has a molecular weight of 63.018 grams per mole. We use dimensional analysis to solve for the moles:

68 grams x (1 mole/ 63.018 grams) = 1.08 moles.

2.) Mass of solvent

To find the amount of solvent, we subtract the amount of solute from the total: 100 grams - 68 grams = 32 grams of water. We then convert this to kilograms.

32 grams (1 kg/ 1000g) = 0.032 kg.

Now, we have everything we need. To find molality, we will divide the moles of solute by the solvent.

molality(m) = 1.08 moles/ 0.032 kg = 33.75 moles/ kg

 

Answer 2

68% HNO3 by mass = 68 g HNO3/100 g solution

To find molality, we need to know moles of HNO3 and kg of water

Moles HNO3 = 68 g x 1 mol/63.01 g = 1.08 moles

kg of water = 100 g - 68 g = 32 g x 1 g/1000 kg = 0.032 kg of water

Molality = moles/kg solvent = 1.08 moles/0.032 kg = 33.8 m

Answer 3

First, what is molality? Molality is moles solute/kg of solute

To get the moles solute, we need to know the molecular mass of HNO₃ = 63.013 g/mol

A concentrated solution of nitric acid is 68% HNO₃ by mass which means that there are 68 grams HNO₃ per 100 grams of solution. If there are 68 grams of HNO₃ in the solution, there must be 32 grams of water, the solvent.

We can set up a calculation as follows:

Molality = Moles HNO₃/kg solute = (68 grams HNO₃/32 grams solute)(1 mole HNO₃/63.013 g)(1000 g/1 kg) = 34 m