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EXPERIMENT 1 Determination of Density of Liquids Using the Westphal Balance Angelo C. Carreon, Kris Juhaina L. Concepcion, Hazel Joy R. Cunamay, Paul Marty T. de Leon and Karen Joy S. Dy Group 3 3A Biochemistry Physical Chemistry I Laboratory ABSTRACT This experiment was performed to study the principles involved in determining the density of liquids using the Mohr-Westphal balance. The sample liquids, which were methanol, absolute ethanol, n-propanol and n-butanol, were subjected to density determination in reference to distilled water. Each density of sample liquids was determined and compared to the actual densities. Analysis of experimental error of each obtained densities was conducted by getting the corresponding percentage errors. The results show that the percentage errors for methanol, absolute ethanol, n-propanol and n-butanol, were 4%, 1%, 9%, and 5% respectively. The relationship of concentration and density should be linear based on investigation in related literatures. However, in the experiment proper using different sucrose solutions of 5%, 10%, 20% and 30% to determine the relationship of concentration and density, it resulted in a non-linear graph due to systematic and human errors committed during the experiment. INTRODUCTION Density is a physical property of matter that expresses a relationship of mass to volume. The more mass an object contains in a given space, the more dense it is. Subsequently different substances have different densities, density measurements are useful means for identifying substances. Density is the ratio of an object's mass to its volume, as shown below. mass Density= volume Equation 1 Because it is a ratio, the density of a material remains the same no matter how much of that material is present. Hence, density is called an intensive property of matter. Mass is the amount of matter contained in an object and is commonly measured in units of grams (g). Volume is the amount of space taken up by a quantity of matter and is commonly expressed in cubic centimeters (cm 3) or in milliliters (ml). Note that 1cm3 is equivalent to 1 ml. Therefore, common units used to express density are grams per milliliters (g/ml) and grams per cubic centimeter (g/cm 3) [1]. Density is directly proportional to the concentration of a solution. If there is greater amount of solute added than the solvent, the concentration increases since the mass of the solution is larger than the increase in volume, thus higher mass causes higher density.

Figure 1. Mohr-Westphal Balance labeled with parts. (1) Foot with adjustment screws; (2) Metal frame with adjustable height; (3) Balance beam with notches; (4) Plummet; (5) Riders. [2]

One method to determine the density of a liquid sample is through the use of MohrWestphal Balance (See Figure 1). The beam of the balance is balanced with the plummet (glass cylinder hanging on a thin platinum wire attached to a hook on the beam) in air using the adjustable screws on the foot. When adjusted, the index pointer on the end of the beam lines up with the point on the frame. The plummet is then completely immersed in the unknown liquid, and the system is

rebalanced, using a series of riders on the nine equally spaced notches on the beam, thus specifying the value of the added mass for each decimal place. This gives the buoyant force of the liquid relative to water, and hence the density, which may be obtained to three decimal places [2]. The ability of an object to "float" when it is placed in a fluid is called buoyant force, and is related to density. If an object is less dense than the fluid in which it is placed, it will float. If it is denser than the fluid, it will sink. 1 Following the mechanism of MohrWestphal balance, this experiment is designed to determine the techniques in using the MohrWestphal Balance to be able to determine the density of liquids, also to understand density as an intensive property of pure liquids; and lastly, to define the relationship between concentration and density of solutions. MATERIALS AND METHODS The following materials were prepared in this experiment: -Mohr-Westphal Balance -pure liquid samples: methanol, ethanol, n-propanol, n-butanol -distilled water -sucrose

absolute

Procedure A. Determination of Density of Liquids Distilled water was poured into the sample container until the volume was enough for the plummet to be immersed. The level of the beam was balanced by placing the riders and by adjusting its distance from the fulcrum. The density of the reference liquid, which is distilled water, was computed using the mass of the riders and their distance from the fulcrum. The same procedure was used in determining the density of the other pure liquid samples.

B. Relationship of Concentration and Density of Sugar Solution The sucrose solutions were made with 5%, 10%, 20%, and 30% weight/volume ratio of sucrose crystals in distilled water. Afterwards, the density of the sucrose solutions were determined using the Mohr-Westphal Balance. The concentration-density relationship was then plotted using a graph. RESULTS AND DISCUSSION The results were discussed in this section with the aid of mathematical equations, tables and graphs. The Mohr-Westphal Balance includes the principle which states that when a body or an object is immersed in a liquid, that object goes through an upward force, or the buoyant force. This force is equal to the weight of the displaced liquid [3]. During the experiment, the principle of Mohr-Westphal balance was observed. After the preparation of different solvents, each value of density were obtained using the Mohr-Westphal balance by putting a rider to a numbered notch on the balance beam. Each rider has its equivalent value: 0.1, 0.01 and 0.001. The experimental density is acquired depending on the rider used and the notch where the rider was placed [4]. Table 1 shows density of liquids obtained from the experiment and the corresponding percentage error of each experimental density sample liquids from the expected. One way to analyze experimental error is with a percentage error calculation. This is useful when you have a single experimental result that you wish to compare with a standard value, and has a formula of % 𝑒𝑟𝑟𝑜𝑟 =

𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝑣𝑎𝑙𝑢𝑒−𝑒𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑣𝑎𝑙𝑢𝑒 𝑒𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑣𝑎𝑙𝑢𝑒

𝑥 100.

The possible reasons of errors were (1) dirt particles added to the liquid in the graduated cylinder, and (2) the slightly false functionality of the Mohr-Westphal balance due to loose screws on the metal frame [5].

Table 1. Density of liquids obtained from the experiment and the corresponding percentage error of each experimental density sample liquids from the expected density.

direct relationship to each other due to the linear graph acquired. On the other hand, blue line segment represented experimental density versus the concentration of sucrose and this should have the same, or slighty similar result to the expected one. However, the result was a nonlinear graph due to errors occurred during the experiment proper. REFERENCES [1] Martha

Marie Day, Ed.D., and Anthony Carpi, Ph.D. "Density." Accessed August 31, 2015. http://www.visionlearning.com/en/library/Ge neral-Science/3/Density/37.

of Solids and Liquids (Pycnometer, Law of Archimedes). Accessed August 31, 2015.

Density, g/mL

[2]Density

http://home.unileipzig.de/prakphys/pdf/Vers ucheIPSP/Mechanics/M-05E-AUF.pdf. [3] Cation, M.D., et al. (2015). Laboratory Manual in Physical Chemistry I. Manila: Philippines. [4] Giancoli, D. (2005). Fluids. In Physics principles with applications (6th ed.). Upper Saddle River, N.J.: Pearson/Prentice Hall. [5] Hewitt, P.G. (2010). Conceptual Physics (11th ed). Pearson Education, Inc. [6] Density of sucrose. Retrieved August 31, 2015 from http://homepages.gac.edu/~cellab/chpts/chpt 3/table3-2.html

1.14 1.12 1.1 1.08 1.06 1.04 1.02 1 0.98 0

10

20

30

40

Concentration, % w/v

Figure 2. Density versus Concentration of Sucrose

Concentration is the total moles of solute per unit volume if it was expressed in molarity, but in this experiment the unit was % weight per volume. Density is mass per unit volume, which means as concentration increases, density should increase too. Hence, the relationship should be directly proportional [6]. In Figure 1, orange line segment represented expected or true density versus the concentration of sucrose which had a