Sample+lab+writeup--AP+Chemistry

__ Title: __ Solution Lab __ Purpose: __ To determine the molarity, molality, density, mole fraction and mass percent composition of a citric acid solution __ Materials: __ Stock citric acid solution 10.00 ml volumetric pipette and rubber bulb 3 evaporating dishes (hot plate—did not lead to conclusion) Analytic balance __ Procedure: __ 1) Label three evaporating dishes 2) Measure and record the mass of each dish using the analytical balance (±.003 g) 3) Deliver 10.00±.04ml of the citric acid solution to each dish using the volumetric pipette and bulb 4) Measure and record the mass of each dish using the analytical balance (±.003 g) 5) Allow each dish to dry in the fume hood for 48 hours 6) Measure and record the mass of each dish using the analytical balance (±.003 g) 7) (Heat gently on a hotplate, Measure and record the mass of each dish using the analytical balance (±.003 g)* Data not taken*) __ Data: __ (copy)
 * Brine lab ||  |||| August 20-22 2008 ||   ||
 * Citric acid || C || 6 ||  || volume ||
 * C6H8O7 || H || 8 ||  || 10.00 ml ||
 * || O || 7 ||  ||   ||
 * ||  || A || B || C ||
 * Data ||  ||   ||   ||   ||
 * mass dish || (g) || 50.5487 || 51.3433 || 37.3654 ||
 * mass w sol’n || (g) || 60.6514 || 61.5195 || 47.5323 ||
 * mass dry || (g) || 51.207 || 52.0089 || 38.0166 ||

__ Calculations: __ || 60.6514 || g- || 50.5487 || g || 10.1027 || g ||  ||   ||   || || 51.207 || g - || 50.5487 || g || 0.6583 || g ||  ||   ||   || || 10.1027 || g - || 0.6583 || g || 9.4444 || g ||  ||   ||   || || 9.4444 || (g) x 1kg/1000g ||  ||   || 0.00944 || kg ||   ||   ||   || || 0.6583 || g / || 192.124 || g/mol || 0.00343 || mol ||  ||   ||   || || 9.4444 || g / || 18.016 || g/mol || 0.52422 || mol ||  ||   ||   || || 10.1027 || g / || 0.01000 || L || 1010.27 || g/l ||  ||   ||   || || 0.0034264 || moles / || 0.01000 || l || 0.34264 || mol/l ||  ||   ||   || || 0.0034261 || mol acid/( || 0.0034264 || mole + || 0.52422 || mole) || 0.01 ||  ||   || || 0.6583 || g / || 10.1027 || g x 100% || 6.51608 ||  ||   ||   ||   || __ Analysis: __ This lab is designed to investigate five ways of expressing the concentration of a solution. The solution used is aqueous citric acid, C6H8O7 (aq). The stock solution was delivered to massed evaporation dishes so that the mass of solution could be found by difference. The volumetric pipette is used for the best precision in delivery of solution (±.04 ml in 10.00 ml is only .4% uncertainty, when properly used.) The evaporation dishes allow evaporation—the dry citric acid (solute) crystals are thereby isolated from the water in which they had been dissolved (solvent). The changes that occurred are: ** C6H8O7 (aq) **** --> **** C6H8O7 (s) ** These are physical changes only. The masses of the dishes with dry crystals were measured in order to find the mass of the crystals __and__ the mass of water lost by difference. The analytic balance is used for the greatest precision throughout. This was performed on three different samples to determine the __variation__ in results that this protocol would allow. The three values in all cases were averaged for the final results. No indications were seen that any trial would not yield valid results. The citric acid solution consists of citric acid molecules dissolved in water. The structural formula of citric acid is given (in Hawley’s condensed chemical dictionary) as HOOCCH2C((OH)(COOH))CH2COOH•H2O. citric acid
 * Calculations ||  ||   ||   ||   ||   ||   ||   ||   ||   ||   ||
 * || C || 6 || 12.01 || 72.06 ||  ||   ||   ||   ||   ||   ||
 * || H || 8 || 1.008 || 8.064 ||  || H || 2 ||   || 2.016 ||   ||
 * || O || 7 || 16 || 112 ||  || O || 1 ||   || 16 ||   ||
 * ||  ||   ||   || 192.124 |||||||| formula mass (FM) of citric acid || 18.016 || FM H2O ||
 * ||  ||   ||   || (g/mol) ||   ||   ||   ||   || (g/mol) ||   ||
 * Mass of solution = mass of dish with solution-mass of dish ||  ||   ||   ||   ||   ||   ||
 * ||  || 10.1027 || 10.1762 || 10.1669 || (g) ||   ||   || >>> || 10.15 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±.04g || (±.4%) ||
 * Mass of citric acid=mass of dish dry-mass of dish ||  ||   ||   ||   ||   ||   ||
 * Mass of citric acid=mass of dish dry-mass of dish ||  ||   ||   ||   ||   ||   ||
 * ||  || 0.6583 || 0.6656 || 0.6512 || (g) ||   ||   || >>> || 0.658 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±.007g || (±1%) ||
 * Mass of water =mass of solution-mass of citric acid ||  ||   ||   ||   ||   ||   ||
 * ||  || 9.4444 || 9.5106 || 9.5157 || (g) ||   ||   || >>> || 9.49 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±.05g || (±.5%) ||
 * Mass of water =mass of water (g) x 1kg/1000g ||  ||   ||   ||   ||   ||   ||   ||
 * ||  || 0.0094444 || 0.0095106 || 0.0095157 || (kg) ||   ||   || >>> || 0.00949 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±.00005kg || (±.5%) ||
 * Moles citric acid=mass citric acid/formula mass ||  ||   ||   ||   ||   ||   ||   ||
 * ||  || 0.0034261 || 0.0034641 || 0.0033892 || (mol) ||   ||   || >>> || 0.00343 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±.00004 mol || (±1%) ||
 * Moles water=mass water/formula mass ||  ||   ||   ||   ||   ||   ||   ||
 * ||  || 0.5242229 || 0.5278974 || 0.5281805 || (mol) ||   ||   || >>> || 0.527 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±.003 mol || (±.5%) ||
 * Density = mass of solution / volume of solution ||  ||   ||   ||   ||   ||   ||   ||
 * ||  || 1010.27 || 1017.62 || 1016.69 || (g/l) ||   ||   || >>> || 1015 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±5 g/l || (±.5%) ||
 * Molarity=moles of solute/volume of solution ||  ||   ||   ||   ||   ||   ||   ||
 * ||  || 0.3426148 || 0.3464141 || 0.3389195 || (M) ||   ||   || >>> || 0.343 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±.004 M || (±1%) ||
 * Molality=moles of solute/kilograms of solvent ||  ||   ||   ||   ||   ||   ||   ||
 * || = || 0.0034261 || mol / || 0.0094444 || kg || 0.36277 || mol/kg ||  ||   ||   ||
 * ||  || 0.3627703 || 0.36424 || 0.3561688 || (mol/kg) ||   ||   || >>> || 0.361 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±.005 mol/kg || (±1%) ||
 * Mole fraction (of solute) = mole solute/ (mole solute + mole solvent) ||  ||   ||   ||   ||   ||
 * ||  || 0.0064932 || 0.0065194 || 0.0063758 ||   ||   ||   || >>> || 0.00646 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±.00008 || (±1%) ||
 * % composition = mass of solute/ mass solution x 100% ||  ||   ||   ||   ||   ||   ||
 * ||  || 6.5160799 || 6.540752 || 6.4050989 |||| % citric acid ||   || >>> || 6.49 || (average) ||
 * ||  ||   ||   ||   ||   ||   ||   ||   || ±.08 % ||   ||
 * H2O(l) **** --> **** H2O(g) **

The –COOH sections make it an acid. The C-O and O-H bonds make it polar, so it does dissolve in water. The solution of the commercial citric acid did show a slight milkiness—perhaps a minor contaminant that did not dissolve. The source also claims that citric acid is efflorescent in dry air, so the crystals were assumed to be anhydrous when dried in Denver. The hotplate would have been used to verify this, but even minor heating caused a color change, raising concerns of loss of product by decomposition. The final mass, therefore, was not taken The solution is a homogeneous mixture. Any sample should contain consistent ratios of molecules of water and acid, of masses of water and acid. The crystals formed small hexagonal structures on the dish. Since the melting point of the acid is given as 153oC, we assume that there was no loss of acid by evaporation. All masses are found by difference. The measured values include (a) the mass of the dish, (b) the mass of the dish and solution, and (c) the mass of the dish and dry crystals. The masses obtained by difference are (b) - (a)= the mass of the solution itself, solute and solvent (citric acid and water) (c) - (a)= the mass of the solute (citric acid) (b) - (c)= the mass __lost__ as it dries, assumed to be mass of the solvent (the water) These masses are obtained in grams. The masses of the water and citric acid are converted to moles using the formula mass of each (192.124g/mol for citric acid, anhydrous, and 18.02 g/mole for water) Additionally, the mass of water is converted to kilograms (1000g=1kg) for the molality calculation. The 10.00 ml of solution is converted to liters for the molarity calculation (1000 ml=1L). Density is defined as the ratio of mass to volume for a sample: ** D=m/V. ** The mass of solution, found by difference is divided by the volume (in liters) giving an average value of 1015 ±5 g/L (1.015±.005 g/mL). This is an expression of concentration useful for a known solute only. Water has a density of 1000 g/l, this solution has a greater density and a more concentrated solution would probably have an even higher density. The density of this solution as a measure of concentration does not compare with an equally concentrated __other__ solute. Aqueous ethanol solutions, for example, __decrease__ in density with greater concentrations of ethanol. Molarity is the most useful expression of concentration. It is defined as the ratio of the number of moles of solute to the volume of solution (in liters). ** M=Mole solute/V (L) solution. ** The moles of solute are divided by the volume of the solution (.01000L). The units obtained, mole/liter (or mol L-1), are abbreviated M (for molarity). This solution was measured at .343 ± .004 M. That is, one full liter of this solution would contain between .339 to .347 moles of citric acid Molality is a similar description of concentration. It is defined as the ratio between the moles of solute and the kilograms of solvent. ** Molality= moles of solute/kg solvent ** The moles of solute are divided by the mass of the solvent, lost while drying, converted to kilograms. The differences include: molality is constant over different temperature ranges while molarity changes as solutions expand and contract with temperature. Molality of different solutes are more comparable, as there is no part of its calculation that is affected by the interaction of __this__ solute with __this__ solvent (as the volume in molarity does). Its weakness is that it is not designed for delivering a given amount of solute. Molality is used when the properties of the solution are of interest, not the amount of solute. This solution was measured as .361 ± .05mol/kg. That is, one full kilogram of water used in this solution would have between .355 to .367 moles of citric acid dissolved in it. The difference in values for the molarity and the molality reflect the interaction between the solute and solvent. If the solute did not affect the volume of the solution, that is, if the volume of the solution was the same as the volume of the solvent— in aqueous solutions, where DH2O=1 kg/L, the molarity and molality are equal. Most solutes, however, form a solution of slightly higher volume than the solvent they dissolve in. This makes the molality slightly greater than the molarity, with more of a difference for more concentrated solutions Mole fraction describes the prevalence of the solute particles related to all of the particles. It is the ratio between moles of solute and total moles in the solution. ** Xsolute=molesolute/ (molesolute + molessolvent) ** (Note: this applies to the mole fraction of solvent as well, where 1-Xsolute=X solvent) The mole of solute are divided by the sum of the moles of the solute and the solvent—the total moles of particles in the sample. Had this been an ionic compound, the mole fraction would have to include the effect of dissociation in the solute. This covalent acid, however, does not dissociate appreciably. There are no units in this relationship. This solution showed X=.00646 ± .00008, that is, between 638 to 654 out of 100000 particles in the solution were solute. This implies that the other 99,346 to 99362 were water molecules. This is also invariant with temperature, but is not directly useful in delivering masses or volumes of solution. The mass percent composition is a very basic way of describing the mass prevalence in any mixture. ** Mass % composition =mass solute / total mass solution x 100% ** The mass of the dry crystals was divided by the total mass of the solution, then converted to a percentage. There are also no units here, but the % symbol is used to show the comparison “out of 100g” This solution was measured as 6.49 ± .08% citric acid. As before, it can be applied to the other part of the mixture. These data indicate between 93.43% and 93.59% water in this solution.

The three trials agreed well. The uncertainty in the volumes and masses compounded leads to the .5% variation among them for density, while minor variations in drying and the uncertainty in the final mass lead to a greater (1%) variation in the other values. __ Sources of error: __ Uncertainty in masses: ±.003 g à neutral effect on results Uncertainty in volumes: ±.04 ml à neutral effect on results Loss of product (not observed): à low values for M, m, X, % comp. Volatile contaminant before (eg. wet dish) (not observed) à higher value for mass solution and water lost, lower value for mass solute à lower calculated values Nonvolatile contaminant (milkiness) à higher calculated values Incomplete drying (not observed) à higher calculated M, m, X, % comp __ Conclusion __ : These data indicate this solution has:
 * D= 1.015 ± .005 g/ml**
 * M=0.343 ± .004 mol L-1 **
 * m=0.361 ± .005 mol kg-1 **
 * X citric acid=0.00646 ± .00008 **
 * Mass%=6.49 ± .08 % citric acid

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