PW No 2: Preparation of Solutions PDF

UNIVERSITY CENTER OF TIPAZA ACADEMIC YEAR: 2024/2025 FACULTY OF SCIENCES LMD-SCIENCE& TECHNOLOGY PW No 2: Preparation of solutions Introduction This practical will introduce you to the fundamental concepts...

UNIVERSITY CENTER OF TIPAZA ACADEMIC YEAR: 2024/2025 FACULTY OF SCIENCES LMD-SCIENCE& TECHNOLOGY PW No 2: Preparation of solutions Introduction This practical will introduce you to the fundamental concepts to prepare solution using dissolution or dilution. Purposes: Upon completion of this lab, the student should be able to:  Know the different ways to express the concentration of a solution.  Proficient calculate molarities for solutions.  Prepare a solution by dissolving.  Prepare a dilute solution from a more concentrated one. Prerequisites  Identify the risks of chemical products.  List the different glassware and their uses.  Recognize the safety instructions to follow in a chemistry laboratory. Theory In this practical work, the preparation of various chemical solutions is a fundamental step that underpins the success of the experiments. Proper preparation of solutions ensures the accuracy of the experimental results and the reliability of the subsequent analyses. This section will detail the procedures for preparing stock solutions, standard solutions, and any specific reagent solutions required for the experiments. The preparation of solutions involves accurately measuring and mixing chemicals in specific proportions to achieve the desired concentration. This process requires a thorough understanding of concepts such as molarity, dilution, and the handling of chemicals to ensure safety and precision. In this session, you will be introduced to the step-by-step procedures for preparing these solutions, including the calculation of concentrations, the use of volumetric flasks and pipettes, and the importance of maintaining a clean and organized workspace. By mastering these techniques, you will lay a solid foundation for the practical application of theoretical knowledge in chemical experiments. Definitions:  Solution in chemistry, a homogenous mixture of two or more substances in relative amounts that can be varied continuously up to what is called the limit of solubility. Solution= Solute + Solvent  Solvent : the substance in which a solute dissolves to produce a homogeneous mixture.  Solute : the substance that dissolves in a solvent to produce a homogeneous mixture.  Concentrations  Molarity: is defined as the number of moles of solute per liter of solution Molarity is also known as the molar concentration of a solution. Unit is (mol/L). CM = n solute / V solution, n = m/M so CM = m / M.V   Mass Concentration (Cm) : is the amount of mass present used to measure the concentration of a solute in a solution ; Unit is (g/L)  Cm = m solute / Vsolution ; Cm= CM*M  Molality : is defined as the number of moles of solute per kilogram of solvent ; is also known as molal concentration ; Unit is (mol/kg) Cmol= n solute / m solvent  Mass Perrcentage: is calculated as the mass of a component divided by the total mass of the mixture, multiplied by 100%. Also Known As: mass percent. A% = m solute / m solution *100  Molar Fraction: defined as the ratio of the number of moles of one component (solute or solvent) to the total number of all the components present in the solution. Xi = n solute / nT solution  Normality: is defined as the number of equivalents per liter of solution. CN=N=neq.g/Vsolution ; CN=Z*CM Z is the number of charged protons for acids and bases. For monoacids (HCl, HCOOH) and monobasics (NaOH, NH3), Z = 1. For diacids (H2SO4) and dibasics (Ba(OH)2), Z = 2. For salts, Z is the number of mineral atoms in their equivalence. Example: Al2(SO4) 2Al+3 + 3SO4 -2 , Z = 2*(+3) = 6. For redox reactions, Z is the number of charged electrons. Example: MnO4 - + 5e+ 8H+ Mn+2 + 4H2O, Z = 5.  Density : it is the amount of mass per unit of volume, density therefore represents, in kg, the mass of a liter of liquid d = mass of liter of liquid / mass of liter of water  Purity: a number expressed as a percentage. It indicates the mass of the chemical species to be dissolved (solute) in 100 g of liquid. Example: For hydrochloric acid, the label indicates d = 1.19 and p = 37% d = 1,19 : - 1 liter of liquid has a mass of 1.19 kg or 1190 g - p = 37% :100 g of liquid in the bottle contains only 37 g (of HCl). Experimental part a- Preparing solution by Dissolution of a chemical species Material Products - Analytical balance, Watch glass, - NaCl Funnel, Volumetric flask 250ml , Wash Bottle - Distilled water Experimental protocol We want to prepare 250 mL of an aqueous solution of Sodium chlorure (NaCl) with molar concentration C1 = 4 × 10-2 mol/L. - First, calculate the molar mass of NaCl which is the mass of a mole of Na plus the mass of a mole of Cl (MNa+MCl = 23 + 35.5 = 58.5 g/mol) and calculate the mass of NaCl (n = mNaCl / MNaCl). - Second, weigh out 0.5 g NaCl. - Third, Place the NaCl in a 250 ml volumetric flask. - Fourth, Add a small volume of distilled water to dissolve the salt. - Fifth, Fill the flask to the 250 ml line. - Sixth, Label the prepared solution. b- Preparing solution by Dilution of a concentrated solution Material Products Funnel, Volumetric flask 100ml , Wash - Distilled water Bottle, volumetric pipettes 5mL, 10mL, - Hydrochloric acid 20mL , pipette (MHCl = 36.5 g.mole-1, ρ = 1.17 kg.l-1, C% = P= 37 %) Experimental protocol We want to prepare 100mL of a solution of Hydrochloric Acid (HCl) (1N).  - First, determine the initial concentration C0 of hydrochloric acid used. - Second, - Determine the volume V0 to be taken from the mother solution. - Third, Take the volume V using the pipette and pour it into the graduated flask. - Fourth, Add distilled water to obtain 100mL. - Fifth, Recap the graduated flask and turn it over several times to homogenize the mixture. - Sixth, Label the prepared solution. Precautions  Concentrated HCl is a very strong acid that will burn your skin instantly upon contact! Wear gloves, goggles, and lab coat.  Do not make the mistake of adding water to concentrated HCl. It has to acid into water. If you do it the wrong way the first drops of water mixing with the concentrated acid will heat up, evaporate, and can send drops of acid flying out of the container onto you.  HClaq above 25% w/w ~ 8M is corrosive  HClaq above 10% ~ 3M is irritant