Given 50 ml of concentrated nitric acid.The density of the concentrated acid is 1.42 g/ml and is 70% HNO3 by wt. Calc. the molality of the conc. acid.

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To calculate the molality of the concentrated nitric acid, we need to first convert the given mass percentage of HNO3 to the grams of HNO3 present in 100 grams of the concentrated acid. Then we can use this value and the density of the acid to calculate the molality.

Let's go step by step:

Step 1: Find the mass of HNO3 in 100 grams of the concentrated acid.
Given that the concentration of HNO3 in the concentrated acid is 70% by weight, we can calculate the mass of HNO3 in 100 grams of the concentrated acid.
Mass of HNO3 = (70/100) * 100 grams = 70 grams

Step 2: Calculate the number of moles of HNO3 in 100 grams.
The molar mass of HNO3 = 1(atomic mass of H) + 1(atomic mass of N) + 3(atomic mass of O) = 1 + 14 + 48 = 63 grams/mol
Number of moles of HNO3 = Mass of HNO3 / Molar mass of HNO3
Number of moles of HNO3 = 70 grams / 63 grams/mol = 1.111 moles

Step 3: Calculate the molality of the concentrated nitric acid.
Molality (m) is defined as the number of moles of solute per kilogram of solvent.
To get the mass of the solvent, we can use the density of the concentrated acid.
Given density = 1.42 g/ml
Volume of the concentrated acid = 50 ml
Mass of the concentrated acid = Volume * Density
Mass of the concentrated acid = 50 ml * 1.42 g/ml = 71 g

Since the mass of the solvent is equal to the mass of the concentrated acid, the molality can be calculated using the formula:
Molality = Number of moles of HNO3 / Mass of the solvent in kg
Molality = 1.111 moles / 0.071 kg ≈ 15.67 mol/kg

Therefore, the molality of the concentrated nitric acid is approximately 15.67 mol/kg.