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Understanding Concentrations in Chemistry: A Comprehensive Guide

Learn about different concentration units like percentage, molarity, parts per million, normality, and more in chemistry. Understand how to express and convert concentrations with examples provided.

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Understanding Concentrations in Chemistry: A Comprehensive Guide

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  1. Concentrations

  2. Concentration units • Concentration of any solution is expressed in terms of the amount of solute dissolved in one unit of the solvent or solution. • The amount of solute dissolved in one unit of the solvent or solution – concentration.

  3. Expressing concentration • Percentage • Wt % of solute = wt of solute/vol. of solution x 100 • Wt % of solute = wt of solute/w of solution x 100 • Vol. % of solute = v of solute/v of solu x 100

  4. Expressing concentration 2. Parts Per Million (ppm) Parts per million of the solute (ppm) = w of solute/w of solution x 106 (if 2g of solute is present in 106 g of solution, its concentration is 2 ppm.

  5. Expressing concentration 3. Molarity (M) Molarity of a solution is defined as the number of moles of the solute dissolved per litre of solution. M = No. of moles of solute/vol. of solution in litres = n/V If W g of a substance having molar mass M, is dissolved in V dm3 of solution, then the molarity (M) = W/MxV mol dm-3

  6. Expressing concentration 4. Formality – no. of formula weights dissolved per litre of the solution: F = no. of formula mass of solute/vol. of solution in litres F = Mass of solute/ formula mass of solute x V of solution

  7. Expressing concentration 5. Normality – defined as the number of equivalents of the solute dissolved per litre of solution. • Normality (N) = no. of equivalents of solute/ vol of solution • N = Mass of solute (W)/ Equivalent mass of solute (E) x Vol of solution (V) • N = W/E x V equiv dm-3

  8. Expressing concentration 6. Molality – defined as number of moles of solute per kg of the solvent. • If solution prepared by dissolving n moles of the solute in W kg of the solvent, then Molality (m) = no. of moles of solute/ mass of solvent in kg = n/W mol per kg or Mass of solute(w)/Molar mass of solute(M) x Mass of solvent in kg or m = w/MxW mol kg-1

  9. Expressing concentration 7. Mole fraction (X) – ratio of the number of moles of that component to the total number of moles of all the components of the solution. Thus, if a solution contains nA moles of A and nB moles of B, then, mole fraction of A (XA) = no. of moles of A/ no. of moles of A + no. of moles of B or XA= nA/nA+ nB. Sum of mole fraction of all the components of a solution = 1.

  10. Changing Molarity to Molality If we know the density of the solution, we can calculate the molality from the molarity, and vice versa.

  11. Activity • What is the molarity of an aqueous solution of ethanoic acid, molar mass = 60, which contains 6.0 g of ethanoic acid in 500 ml of solution? • Calculate the molality of one litre of 93% H2SO4 solution (W/V). The density of the solution is 1.84 g per ml.

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