What is the molar mass of nitric acid, HNO3, expressed in grams per mole using standard atomic masses?

Introduction / Context:
Molar mass is a basic quantity in chemistry used to convert between moles and grams. For acids such as nitric acid, HNO3, knowing the correct molar mass is essential for preparing solutions of known concentration, performing titrations and solving stoichiometry problems. This question asks you to calculate or recall the molar mass of HNO3 using standard atomic masses from the periodic table and select the correct value from the options provided.

Given Data / Assumptions:

• Chemical formula of nitric acid is HNO3.
• Approximate atomic mass of hydrogen (H) is 1 g/mol.
• Approximate atomic mass of nitrogen (N) is 14 g/mol.
• Approximate atomic mass of oxygen (O) is 16 g/mol.
• We assume standard atomic masses as used in introductory chemistry calculations.

Concept / Approach:
To find the molar mass of a compound, we multiply the atomic mass of each element by the number of atoms of that element in the formula and then add all these contributions. For HNO3, there is one hydrogen atom, one nitrogen atom and three oxygen atoms per molecule. The approach is straightforward: compute 1 for hydrogen, 14 for nitrogen and 3 * 16 for oxygen, then sum the results. This gives the mass of one mole of nitric acid molecules in grams.

Step-by-Step Solution:
Step 1: Identify the elements in HNO3: hydrogen (H), nitrogen (N) and oxygen (O). Step 2: Count the number of each atom in the formula: 1 hydrogen, 1 nitrogen and 3 oxygen atoms. Step 3: Use the approximate atomic masses: H = 1 g/mol, N = 14 g/mol and O = 16 g/mol. Step 4: Compute the total mass contribution: hydrogen contributes 1 * 1 = 1 g/mol, nitrogen contributes 1 * 14 = 14 g/mol and oxygen contributes 3 * 16 = 48 g/mol. Step 5: Add these values: 1 + 14 + 48 = 63 g/mol. Step 6: Therefore, the molar mass of nitric acid, HNO3, is 63 g/mol.

Verification / Alternative check:
Using more precise atomic masses, such as H = 1.008, N = 14.01 and O = 16.00, the calculation becomes roughly 1.008 + 14.01 + 48.00 = 63.018 g/mol, which rounds to 63 g/mol for typical classroom use. Many reference tables list the molar mass of nitric acid as approximately 63.0 g/mol, confirming the calculation. Industrial data sheets for concentrated nitric acid solutions also use this molar mass in preparing solutions, further verifying the value.

Why Other Options Are Wrong:
Option B, 53 g/mol, would imply a much lower oxygen contribution and does not match the three oxygens in HNO3. Option C, 48 g/mol, matches only the oxygen contribution but ignores hydrogen and nitrogen, so it is incomplete. Option D, 52 g/mol, does not correspond to any reasonable combination of atomic masses in HNO3. Option E, 73 g/mol, is too high and would require heavier elements or more atoms than are actually present. Only option A, 63 g/mol, corresponds to the correct sum of atomic masses for one mole of nitric acid molecules.

Common Pitfalls:
Learners sometimes forget to multiply the oxygen atomic mass by three, treating HNO3 as if it had only one oxygen atom. Another mistake is to confuse HNO3 with other nitrogen oxides or oxyacids like HNO2 or N2O5, leading to incorrect atom counts. To avoid these errors, always write the formula clearly, count each atom carefully and multiply atomic masses by the subscripts. This method works reliably for any compound and helps develop good habits for more complex stoichiometric calculations.

Final Answer:
The molar mass of nitric acid, HNO3, is 63 g/mol, obtained by adding the contributions of one hydrogen, one nitrogen and three oxygen atoms.