In solution chemistry, the number of moles of solute present in exactly one litre (1 dm^3) of solution is called the ______ of the solution.

Introduction / Context:
This question tests a fundamental concept in solution chemistry: the different ways of expressing concentration. It asks for the term used when we express concentration as the number of moles of solute per litre of solution. Getting these definitions correct is crucial for solving stoichiometry problems in reactions that take place in solution.

Given Data / Assumptions:

• The definition involves moles of solute.
• The volume considered is one litre (1 dm^3) of the final solution, not of pure solvent.
• Possible terms include normality, molarity, molality and formality.
• We consider standard definitions as used in introductory chemistry.

Concept / Approach:
Molarity (symbol M) is defined as the number of moles of solute dissolved in one litre of solution. Molality (symbol m) is the number of moles of solute per kilogram of solvent. Normality is based on gram equivalents of solute per litre of solution. Formality is similar to molarity but used for ionic solutes that dissociate. Therefore, when the question refers specifically to moles per litre of solution, the correct term is molarity.

Step-by-Step Solution:
1) Read the definition in the question: number of moles of solute in one litre of solution.2) Recall the definition of molarity: Molarity = moles of solute / volume of solution in litres.3) Compare with molality: Molality = moles of solute / mass of solvent in kilograms, which is different because it uses solvent mass, not solution volume.4) Compare with normality: Normality = gram equivalents of solute / litre of solution, which is based on equivalents rather than moles.5) Compare with formality: Formality is sometimes used for ionic solutes and is based on formula units per litre, but the standard school level answer for moles per litre is molarity.6) Therefore, the term that exactly matches the given definition is molarity.

Verification / Alternative check:
Textbooks and laboratory manuals consistently define molarity as the primary concentration unit for reactions in solution. For example, a 1 M solution of sodium chloride contains 1 mole of NaCl in 1 litre of solution. When performing titrations, chemists often refer to molar solutions such as 0.1 M HCl or 0.2 M NaOH, clearly using moles per litre as the basis. This reinforces that the correct term is molarity and not any of the other alternatives.

Why Other Options Are Wrong:
Normality: Uses equivalents per litre, not moles per litre, and depends on the reaction type (acid base, redox and so on).
Molality: Uses moles of solute per kilogram of solvent, so the volume of solution does not appear in the definition.
Formality: Related concept for ionic solutes but not the standard name for moles per litre; it is used less commonly than molarity.
Equivalent concentration: A descriptive phrase that relates more closely to normality than to molarity.

Common Pitfalls:
Students sometimes confuse molarity and molality because the names sound similar. A helpful memory aid is that molarity starts with letter M and involves volume in litres (think of measuring in a measuring flask), whereas molality starts with letter m and involves mass of solvent. Another trap is mixing up molarity and normality; remembering that normality depends on equivalents and the specific reaction helps keep the two concepts separate. Whenever you see moles per litre of solution, think of molarity.

Final Answer:
The number of moles of solute present in one litre of solution is called the molarity of the solution.