Determination of Densities

Maria Zenaida B. Lu

Camille Anne M. Rivera

De La Salle University – Dasmariñas

Dasmariñas City, Cavite, Philippines

ABSTRACT

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The densities of different kinds of solids and liquids were determined by finding their masses and volumes. For liquids, a certain volume was indicated -- 50mL water and 5mL each for alcohol, coconut oil, and unknown liquid sample. The masses then were measured by weighing the container first, then the container with the sample, and subtracting the acquired data to get the masses of each liquid sample. For solids, the masses were obtained by weighing them directly on a triple beam balance or weighing a watch glass first, then the watch glass plus the solid. There are two categories for the solids – regularly-shaped and irregularly-shaped solids. The regularly shaped solid was a rectangular block of wood. Measuring the length, width and height of the block and multiplying them to each other will yield the volume. Next, a marble was used for the spherical solid. Finding the volume of a sphere will require you to get the circumference first, and then through computation, find the radius then volume. Lastly, pebbles were used for the irregularly-shaped solids. Volumes of this kind of solids are obtained through water displacement. Once all of the masses and volumes are measured, the densities are computed using the formula D=m/v where, m is the mass of the sample in grams and v is the volume of the sample in milliliters. After the experiment, the true values of the densities were revealed allowing the comparison of experimental density to the known density.

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INTRODUCTION

Density is a physical and intensive property of matter which describes how close the atoms of an object are to each other. It can also be defined as the ratio of mass per unit volume. (1) The SI-derived unit for density is the kilogram per cubic meter (kg/m^3). This unit is awkwardly large for most chemical applications. Therefore, grams per cubic centimeter (g/cm^3) and its equivalent, grams per milliliter (g/mL), are more commonly used for solid and liquid densities. (2) Water, with a density of 1.00 g/mL, was used as the standard for determining densities. In this experiment, we can determine the densities of some commonly used substances.

MATERIALS AND METHODS

The samples were categorized as liquids and solids. Under the liquid samples were the subcategories of water and liquids other than water. Subcategories for solid samples were regularly-shaped and irregularly-shaped. All in all there were seven samples, namely water, alcohol, coconut oil, wooden block, marble, pebbles, and an unknown liquid sample. A triple beam balance was used to determine the masses of all the samples. For samples needing a container, the first to be weighed is the clean and dry container (watch glass or graduated cylinder) then followed by weighing the container with the sample. To get the mass of the sample, the mass of the container was subtracted to the mass the container with the sample. In determining the volume, four techniques can be used – measuring through graduated cylinder (direct measurement), measuring the length, width and height, measuring the circumference, and lastly, water displacement. All except the first method for determining volume require computation. Liquids were measured through the first technique. The regularly-shaped solid and spherical solid’s volumes were later acquired using the formulas in Table 1. Irregularly-shaped solid require the use of water displacement to find the volume. Water displacement is a simple technique where a specific volume or water was measured in a graduated cylinder first. Then the solid object was immersed in the water and the final volume was measured. The amount of the final volume subtracted to the initial volume was the displaced water and the total volume of the immersed solid. After all

the raw data had been gathered, the derived mass and volume of the solid and liquid samples were determined indirectly using the formulas as shown in Table 1.

Regularly-shaped solid

V = L x W x H ; L = Length

W = Width

H = Height

Spherical solid

V = ;

r = radius

c = circumference

Table 1. Formulas for Volume

RESULTS AND DISCUSSION

Density plays an important role in the concepts of buoyancy and purity. In buoyancy, the basis is the density of water which is 1g/mL; anything that has a lower density would float and anything with a higher density would sink in water. In purity, the density can tell how much of a certain substance is in the matter. Normally, substances with higher densities sink in substances with lower densities. Experimental results of four liquid samples and three solid samples are summarized in tables 2 and 3 respectively.

Table 2 shows the data gathered and computed for all the liquid samples of volumes 50 mL for water and 5 mL for all the other liquid samples.

Water Alcohol Coconut Oil Unknown Sample

Mass of container 94.50 g 25.20 g 25.20 g 25.20 g

Mass of container with liquid

145.50 g 29.40 g 29.10 g 29.70 g

Mass of liquid 51.00 g 4.20 g 3.9 g 4.5 g

Volume of liquid 50.0 mL 5.00 mL 5.00 mL 5.00 mL

Experimental Density

1.02 g/mL 0.840 g/mL 0.78 g/mL 0.9 g/mL

Known Density 1.00 g/mL 0.79 g/mL 0.90 g/mL 0.81 g/mL

Percentage error 2.00% 6.33% 13.33% 11.11%

Table 2. Densities of Liquids

The obtained experimental densities of the samples were compared to the known density to find its percentage error. The identity of the unknown liquid sample was later revealed since it had an experimental density of 0.9 g/mL, to which the closest density observed in kerosene which has a known density of 0.81 g/mL. Based on the results, it is observed that higher volumes show the least percentage error.

Table 3 specifies the experimental densities of regularly-shaped solid samples and irregularly-shaped solid samples.

Irregularly-shaped solid/s Regularly-shaped solid/s

Pebbles Wooden block Marble

Mass of container 33.60 g n/a 46.10 g

Mass of solid with container

46.10 g n/a 52.00 g

Mass of solid 12.50 g 74.5 g 5.90 g

Dimensions (length, height, width)

n/a12.45 cm x 6.33 cm x

1.75 cmn/a

Circumference n/a n/a 5.20 cm

Volume 4.0 mL 137.91 mL 2.37 mL

Density 3.13 g/mL 0.38 g/mL 2.19 g/mL

Table 3. Densities of Solids

Though not directly, the volumes of the two regularly-shaped solids were determined through derivation using the formulas in Table 1. The volume of the irregularly-shaped solid was determined by using the water

displacement method. By applying the law on buoyancy, anything which has a greater density than water (1.00 g/mL) will sink on it. Based on the findings in Table 3, only the wooden block would float in water.

REFERENCES

(2) Chang, R. & Overby, J. 2011. General Chemistry: The Essential Concepts. 6th edition. USA: McGraw-Hill

(1) Masterton, W. L. & Hurley, C. N.1999. Chemistry: principles and reactions. 5th edition. USA: Thomson Inc.

Samonte, J. L. & Figueroa L. V. 2007. General Chemistry Laboratory Manual. 3rd edition. Philippines: C & E Publishing