Tp Density Of Sucrose Soltns

FINDING A CONCENTRATION - SPECIFIC GRAVITY CURVE Introduction A solution is a homogeneous (see-through) system that can be split into two or more different substances. Solutions are prepared by mixing and stirring a substance into another one (most frequently a liquid) until just one phase can be seen. They can show different concentrations, that is different proportions. The concentration of a solution can be given in different ways for example as percentage mass to mass (% m / m). If an aqueous salt solution is said to be 15 % m / m then 100 gram of this solution will be formed by 15 gram of salt and 85 gram of water. If it said to be 15 % m / v (the so called percentage mass to volume) then 15 gram of salt will be in 100 cm3 of solution (it doesn’t mean that necessarily there are 85 cm3 of water). Density is defined as the mass of the unitary volume of a material. The unit of density more frequently found in a lab is g / cm3. The density of a solution changes with changing composition. It cannot be calculated adding the volumes of the substances that have been mixed because, although the masses are added, their volumes are not. This is because as the solution is being formed, the particles of the solute mingle with the particles of the liquid moving around through the “holes” between its particles. In addition, they become solvated: the solvent’s molecules cluster and stick around the particles of the solute maybe “shrinking”. That is the reason why the density of solutions must be measured experimentally. A word about the units: be careful! Although both units are apparently the same (g/cm3) they have absolutely different meanings: density is the total mass of one cm3 of solution a very different thing to the mass to volume concentration, the mass of solute contained in that cm3. Many times the food industry has to deal with sucrose (sugar) solutions, at some stage of a preparation. To tell whether these solutions contain the proper amount of sugar, a sample is taken to the lab where it can be analysed in different ways. Some chemical analyses can be rather time consuming but the determination of the density of the sample shows accurate enough and constitutes a very simple and quick method to assess the concentration of sugar in the syrupy solution. A density meter is dipped into the temperature controlled sample and the reading is changed to concentration of sucrose in the solution using a calibration curve. Aims  To prepare solutions of known composition  To measure their densities (relative to water) at 20 ± 2 ºC  To draw a calibration curve for the assessment of sucrose in aqueous solutions by means of density determinations Materials 250 ml beaker, stirring rod, 250 ml volumetric flask, funnel, wash bottle, spatula, watch glass, scale. Distilled water, sucrose. Procedure Note: the masses of sucrose to be weighed will be different for each of the groups and the data will be shared.

1- Tare a 250 ml (250 cm3) volumetric flask (with its stopper). Record your measurement in the table. 2- Weigh in a watch glass X g of sucrose. (Teacher will tell the group how much) Record your measurement. 3- Collect the sucrose into the beaker and add 60-80 ml of distilled water. 4- Stir to dissolve. 5- Transfer into the volumetric flask 8 use a funnel to avoid losses) 6- Wash the beaker repeatedly squirting small amounts of distilled water with the wash bottle transferring the washings into the flask. Use the glass rod to aim the liquid as you pour it. 7- Be sure the total volume is less than 250 ml!!! 8- Stopper the flask and homogeneise turning it up and down several times. 9- Carefully fill to the mark using the wash bottle or a pipette. 10- Homogeneise once again. 11- Weight the flask with its contents. Record your measurement. 12- Calculate the concentration in % m/m and the density as g / cm3 and record your results. 13- Ask the other groups for the results that they have found during their experiments. Record all the information in a chart writing them in order of increasing concentration. 14- Plot density against concentration in graph paper Exp. Nr

Mass sugar (g)

Volume Conc. Mass flask solution flask (cm3) (% m/v) (empty ) (g)

Collected data of all groups Conc %m/v Dens. g/cm3 Stick plot here

Mass Mass Density flask + solution solution solution (g) (g/cm3) (g)