General Chemistry
The annual mole fair serves as an interesting platform to discuss the different ways to measure the concentration of a solution. Concentration is the amount of a compound, called the solute, that is dissolved in another substance, called the solvent. Various calculations are used to measure concentration, such as percent composition, which is based on mass and calculates the percentage of a solution's mass that comes from the solute. Another measure is mole fraction, represented by the capital letter X, which indicates the portion of total moles in a solution made up of a specific component.
Molarity represents the number of moles of a solute per one liter of solution, and is indicated using brackets around a value. The molarity of a solution can also be denoted with a capital M. Molality is the measure of concentration that represents the moles of solute per kilogram of solvent. The mass of one equivalent of a substance is known as equivalent weight or gram equivalent. Lastly, normality is the number of equivalents per one liter of a solution, and can be calculated by dividing the number of equivalents for a component by the liters of solution.
Lesson Outline
<ul> <li>Concentration: Amount of solute dissolved in a solvent</li> <li>Percent Composition: Based on mass, percentage of solution's mass from the solute</li> <ul> <li>Calculation: (mass of solute / mass of solution) x 100%</li> <li>Tip: if possible, measure the solution and solute before combining</li> </ul> <li>Mole Fraction (X): The portion of total moles in a solution made of a specific component</li> <ul> <li>Calculation: moles of component A / total moles of all species</li> <li>Sum of mole fractions in a solution will always add up to one</li> </ul> <li>Molarity: Number of moles of solute per one liter of solution (moles / liters)</li> <ul> <li>Indicated using brackets around a value</li> <li>Denoted with a capital M, read as "molar"</li> <li>Dilution equation: Mi * Vi = Mf * Vf</li> </ul> <li>Molality: Moles of solute per kilogram of solvent (moles / kg)</li> <ul> <li>Only measure that doesn't use the whole solution in the denominator</li> </ul> <li>Equivalent: Amount of a substance that will react with or produce one mole of the reactive species in a chemical equation</li> <ul> <li>Example: one mole of HCl reacts with one mole of NaOH, but only 0.5 mole of H2SO4 needed</li> <li>Equivalent weight or gram equivalent: mass of one equivalent</li> </ul> <li>Normality: Number of equivalents per one liter of a solution (equivalents / liters)</li> <ul> <li>Specific to a reaction (e.g., acid-base reactions with polyvalent species), value is not consistent</li> </ul> </ul>
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FAQs
There are several measures of concentration used to describe the components of a solution, including percent composition, mole fraction, molarity, molality, and normality. Each measure provides a different aspect of how concentrated the components of a solution are, depending on the context and calculation requirements.
Percent composition is calculated by dividing the mass of a specific component (solute) by the total mass of the solution and then multiplying the result by 100 to give a percentage. The formula is: % Composition = (mass of solute / mass of the solution) × 100. This measure indicates the proportion of a specific component in a solution and is widely used in various branches of chemistry and medicine.
Molarity and molality are both measures of concentration, but they are calculated differently. Molarity is defined as the number of moles of solute per liter of solution, so its unit is moles/L or M. Molality, on the other hand, is defined as the number of moles of solute per kilogram of solvent, so its unit is moles/kg or m. While molarity depends on the volume of the solution, molality depends on the mass of the solvent, hence molality is not affected by changes in temperature or pressure.
The dilution equation is used to calculate the concentration of a solution after it has been diluted by the addition of more solvent. The equation is: Mi * Vi = Mf * Vf, where Mi and Mf are the initial and final concentrations of the solute, and Vi and Vf are the initial and final volumes of the solution. This equation is commonly used in laboratories to prepare solutions with specific concentrations from stock solutions of higher concentrations.
Normality is a measure of concentration that takes into account the chemical reactivity of a solute in a solution. It is defined as the number of gram equivalents of solute per liter of solution. To calculate normality, you would use the formula: Normality = (mass of solute / equivalent weight) / volume of solution in liters.Equivalent weight is the mass of a substance that can react with or displace a specific amount of another substance. It is calculated by dividing the molecular weight of the solute by the number of equivalents it can produce. The gram equivalent is the mass of solute required to provide one equivalent of the substance.Normality is particularly useful when working with acid-base reactions, as it accounts for the number of protons (or hydroxide ions) that can be donated (or accepted) by the solute during a reaction.
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