In solution chemistry, the symbol "M" written after a concentration value (for example, 0.10 M HCl) most commonly stands for which unit of concentration?

Introduction / Context:
In solution chemistry, different units describe how much solute is present in a given amount of solvent or solution. Laboratory bottles, textbooks and exam questions often use symbols such as M, m and N to denote different concentration units. Knowing what each symbol stands for is essential for correctly interpreting data and performing stoichiometric calculations involving solutions. This question focuses on the meaning of the capital letter M.

Given Data / Assumptions:

• The symbol M appears after numerical values, such as 0.10 M HCl.
• We are asked what unit of concentration M represents.
• Options mention molarity, molality, normality, mole fraction and mass percent.
• We assume standard usage in introductory chemistry courses.

Concept / Approach:
Molarity is defined as moles of solute per litre of solution. It is denoted by the symbol M, so 1 M means 1 mole per litre. Molality is moles of solute per kilogram of solvent and is often denoted by a lower case m. Normality uses N and refers to equivalents per litre. Mole fraction is a ratio without units, and mass percent is expressed as a percentage. Therefore, when you see a capital M following a number and a formula, it indicates molarity.

Step-by-Step Solution:
Step 1: Recall that a 1 M solution contains 1 mole of solute in each litre of final solution.Step 2: Note that chemists write this as 1 M, pronounced "one molar".Step 3: Recognise that the symbol for molality is lower case m, not capital M.Step 4: Understand that normality uses N, not M.Step 5: Mole fraction is typically represented by the symbol x and has no units.Step 6: Mass percent is given as a percentage and does not use the letter M as a unit symbol immediately after a number.Step 7: Therefore, M stands for molarity, moles of solute per litre of solution.

Verification / Alternative check:
Look at labels on standard laboratory solutions, such as 0.1 M NaOH. Manuals and calculations that use these solutions explicitly state that molarity equals moles per litre. Problems involving dilution also use the equation M1V1 = M2V2, where M represents molarity. These repeated uses in curriculum and laboratory practice confirm that M is the symbol for molarity.

Why Other Options Are Wrong:
Option b, molality, uses a different symbol and depends on mass of solvent, not on solution volume. Option c, normality, refers to equivalents per litre and uses N. Option d, mole fraction, is a ratio and not typically labelled with M. Option e, mass percent, is expressed in percent and does not use M as a symbol for the concentration unit.

Common Pitfalls:
A common confusion is between molarity and molality because their names sound similar. Another pitfall is misreading a lowercase m as M or vice versa in handwritten notes. To avoid mistakes, remember that molarity uses the capital letter M and involves volume of solution, while molality uses lowercase m and involves mass of solvent. Keeping these distinctions clear helps ensure accurate interpretation of solution data.

Final Answer:
In chemistry, the symbol "M" after a concentration value stands for molarity, meaning moles of solute per litre of solution.