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Mastering Moles and Molar Concentration: From Volume to Moles
Unlock the secrets of moles and molar concentration! Learn to convert volume to moles, understand concentration formulas, and excel in chemistry calculations. Perfect for students tackling stoichiometry and solution chemistry.

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Now Playing:Stoichiometry and molar volume – Example 0a
Intros
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  1. Expanding our moles calculations
  2. Expanding our moles calculations
    Recap - moles, molar volume and concentration
  3. Expanding our moles calculations
    Calculating concentration of a solution
Examples
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  1. Find the number of moles and concentration of substances used in chemical reactions.
    1. Calculate the number of moles in 100 mL of 0.2M HCl (aq)_{(aq)}

    2. Calculate the concentration of a solution of 1.7 litres of water with 0.75 moles of HCl dissolved in it.

    3. Calculate the new concentration of this solution when 3.4 extra litres of water are added to the solution.

Introduction to stoichiometry
Jump to:NotesConceptFAQsPrerequisites
Notes
In this lesson, we will learn:
  • To relate the mole concept to amount of substance in aqueous solutions.
  • To calculate the molarity of aqueous substances.
  • To calculate amounts of substance from titration problems.

Notes:

  • We saw in Moles, mass and gas calculations that for practical reasons, gas is usually measured in volume as opposed to mass which is used for solids. When we fix (keep constant) the temperature and pressure, at STP or RTP, the volume that one mole of an ideal gas occupies is also fixed, or constant.
    • At standard temperature and pressure (STP) it is 22.4 litres per mole (L mol-1).
    • At room temperature and pressure (RTP) it is 24 litres per mole (L mol-1).

    Like gases, when it comes to aqueous substances (something dissolved in water), mass and volume are not used. Instead, concentration is used.

  • Concentration effectively means “how much stuff in how much space?” which is why in chemistry concentration is measured in moles per litre, or moles per cubic decimetre (the value is equal).
    Sometimes the term molarity is used. Molarity means concentration, for chemists – molarity is the number of moles of a chemical per amount of volume.
    • Units of concentration are abbreviated “M”. It means moles per litre, written mol / L or mol L-1 or moles per cubic decimeters, written mol/dm3 or mol dm-3.
    • Square brackets, e.g. [HCl] are used to show the concentration of a chemical. For example, you could write [HCl] = 0.1 mol dm-3 which means there is 0.1 mole of HCl per 1 litre of this solution.

    You can use c=c = nV\large \frac{n}{V} to find concentration, where n = number of moles and VV is volume (in liters, L, or cubic decimeters, written dm3). You can then re-arrange for n=c∗vn = c * v.

  • Because concentration is measured in one unit per another unit (moles per litre), using the conversion factor method will not cancel your units like the examples with mass or volume. It is first easiest to separate the two unit conversions, combining them at the end.
    • For example, find the concentration of a solution of 54g of solid NaOH pellets being dissolved in 1500 mL of water.
      Our target units are moles per litre: we will find moles using the mass and molar mass first, then find litres using the mL value given and then we will divide the moles by litres:

    54 g NaOH ∗ g \, NaOH \,* \, 1 mol NaOH40 g NaOH\large \frac{1 \, mol \, NaOH} {40 \, g \, NaOH} = = 1.35 mol NaOHmol \, NaOH

    1500 mL NaOH ∗ mL \, NaOH \,* \, 1 L NaOH1000 mL NaOH\large \frac{1 \, L \, NaOH} {1000 \, mL \, NaOH} = = 1.57 L NaOHL \, NaOH

    Using C=n/vC = n/v we can now find the concentration by dividing moles by volume:

    1.35 mol NaOH1.5 L NaOH\large \frac{1.35 \, mol \, NaOH} {1.5 \, L \, NaOH} = = 0.9 M NaOHM \, NaOH

  • Using concentration to find the number of moles is very useful for knowing the amounts of substance in titration experiments.
    A titration is an experiment used to find out the unknown concentration of an acid by reacting it with a base of known concentration, or vice versa (unknown base with known acid). We call the solution of known concentration a standard solution. The concentration of this is known precisely.
    For an acid of known concentration reacting with a base of unknown concentration:
    • A titration experiment runs until the number of moles of acid equals the number of moles of base, which is found by a colour change using an indicator.
    • The known acid concentration and volume of acid is used to find the number of moles of acid, which is used to find the number of moles of the base of unknown concentration.
      Look for the molar ratio from the chemical equation, for example if acid A and base B react in the equation 2A + B   \, →  \, C + H2O to make salt C, there is a 2:1 molar ratio. The moles of A will be twice the moles of B.
    • You use the number of moles found and the volume of the unknown concentration to find the concentration of the unknown substance.

    In the exercises of this lesson we will practice worked calculations using data from titration experiments, using the equation for concentration and unit conversions shown above. The detailed process of titration experiments is looked at in Acid-base titration

  • Be careful with units of concentration, converting units if you need to. Volume is often given in mL but concentration is measured in moles per litre or moles per cubic decimeter, which has the same value. Dividing by 1000 converts from mL to L.
Concept

Introduction

Moles and molar concentration are fundamental concepts in chemistry, essential for understanding stoichiometry and performing accurate calculations. A mole represents a specific number of particles (6.022 x 10^23), while molar concentration describes the amount of solute in a given volume of solution. The introduction video provides a clear and concise explanation of these concepts, serving as a crucial foundation for students embarking on their chemistry journey. By grasping moles and molar concentration, learners can effectively tackle more complex chemical problems and reactions. These concepts are indispensable in various chemistry calculations, including balancing chemical equations, predicting reaction outcomes, and determining product yields. Mastering moles and molar concentration is vital for success in chemistry, as they form the basis for understanding stoichiometric relationships and solution chemistry. The video's significance lies in its ability to demystify these abstract concepts, making them accessible and applicable to real-world chemical scenarios.

FAQs

Here are some frequently asked questions about moles and concentration:

1. How do you convert mL to moles?

To convert mL to moles, you need to know the concentration of the solution. Use the formula: moles = (volume in L) × (concentration in mol/L). First, convert mL to L by dividing by 1000, then multiply by the concentration.

2. What is the relationship between concentration and molar concentration?

Molar concentration is a specific type of concentration that expresses the number of moles of solute per liter of solution. It's typically measured in mol/L or M (molarity). Other concentration units can be converted to molar concentration if the molecular weight of the solute is known.

3. How do you calculate the number of moles from volume?

To calculate moles from volume, use the formula: n = C × V, where n is the number of moles, C is the concentration in mol/L, and V is the volume in liters. Make sure to convert the volume to liters if it's given in mL.

4. What is the concentration formula c1v1 = c2v2?

This formula is used for dilution calculations. It states that the product of the initial concentration (c1) and volume (v1) equals the product of the final concentration (c2) and volume (v2). It's useful for determining the concentration or volume after diluting a solution.

5. How do you convert percentage concentration to moles?

To convert percentage concentration to moles, first convert the percentage to a decimal (e.g., 5% = 0.05). For w/v percentages, this decimal represents grams per mL. Multiply by the total volume in mL to get grams, then divide by the molecular weight to get moles. For v/v percentages, additional density information may be needed.

Prerequisites

Understanding moles and molar concentration is a fundamental concept in chemistry that builds upon several key prerequisite topics. While there are no specific prerequisite topics provided for this article, it's important to recognize that a strong foundation in basic chemistry principles is essential for grasping these concepts effectively.

Moles and molar concentration are central to many chemical calculations and processes. To fully comprehend these ideas, students should have a solid understanding of atomic structure, chemical formulas, and basic mathematical skills. These foundational concepts provide the necessary framework for exploring the world of moles and molar concentration.

Atomic structure knowledge helps students understand how atoms combine to form molecules and compounds, which is crucial when dealing with moles. Familiarity with chemical formulas allows for the accurate representation of substances and their relative quantities in chemical reactions. Additionally, basic mathematical skills, including algebra and unit conversions, are indispensable for performing mole-related calculations and determining molar concentrations.

The concept of moles serves as a bridge between the microscopic world of atoms and molecules and the macroscopic world of measurable quantities. It allows chemists to relate the number of particles in a substance to its mass and volume. This relationship is fundamental in stoichiometry, solution chemistry, and many other areas of chemical analysis.

Molar concentration, often expressed as molarity, is a measure of the amount of solute dissolved in a given volume of solution. This concept is crucial for understanding solution properties, reaction rates, and equilibrium processes. It builds upon the idea of moles and introduces the importance of volume in chemical calculations.

By mastering these prerequisite topics, students can more easily grasp the intricacies of moles and molar concentration. They will be better equipped to solve complex chemical problems, predict reaction outcomes, and understand the behavior of substances in various chemical environments.

As students progress in their chemistry studies, they will find that moles and molar concentration are recurring themes that connect various aspects of the subject. These concepts are essential for advanced topics such as acid-base chemistry, thermodynamics, and chemical kinetics. Therefore, a strong foundation in the prerequisites will not only aid in understanding moles and molar concentration but also pave the way for success in more advanced chemistry courses.

In conclusion, while specific prerequisite topics were not provided, it's clear that a solid grounding in basic chemistry principles is vital for mastering moles and molar concentration. Students should focus on strengthening their understanding of atomic structure, chemical formulas, and mathematical skills to build a robust foundation for these important chemical concepts.