Density.docx - Density Introduction In this lab you will...

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Density Introduction: In this lab you will have the opportunity to use the electronic balances, graduated cylinders, and volumetric pipettes to determine the densities of different liquids and solids using a variety of techniques. During this experiment, focus on using the proper number of significant figures in all of your measurements and in ALL of the calculations. Background: Density is calculated by dividing a mass by a volume, as shown below in Equation 1. Equation 1: m D = V Although the “proper” SI unit for density is kg/m 3 , in chemistry, the unit that we generally use to measure mass is the gram (g); volumes of solids and liquids are measured in cubic centimeters (cm 3 ) or milliliters (mL). Therefore, your units for density are either g/cm 3 or g/mL. Since one milliliter is equal to one cubic centimeter, 1 g/cm 3 is equivalent to 1 g/mL. Determining the Density of a Liquid: A container must be used to hold the liquid that you wish to weigh. This container will either need to be pre-weighed to determine its mass. Next, the liquid is added to the container and the combined mass is determined. A simple subtraction yields the mass of the liquid. To obtain the volume of the liquid, an accurate measuring device is required. This could be either a graduated cylinder or a pipette (but not a beaker!). When using the graduated cylinder, keep in mind that it should be read as accurately and precisely as possible. Before you start, look at the graduated cylinder and decide what your “best guess” is for that particular cylinder. Then, making sure you are at eye-level with the level of the liquid, read the very bottom of the meniscus (the meniscus is the curve or arc-shaped appearance of the water). A second method is to use a volumetric pipette. If the pipette is filled to the correct mark (that is – bottom of the meniscus again), it will deliver a precise and specific volume of that liquid. Determination of the Density of a Solid: To obtain the mass, simply weigh the object on the scale. Some solids may need to be weighed in a weighing boat or a beaker – if this is the case, then remember to account for the mass of the object used to hold the solid. To obtain the volume of the solid, one of the two following procedures should be selected based on the appropriateness. If the object has a uniform shape with measurable dimensions, then it is possible to use a ruler to find the volume. Simply measure the dimensions in centimeters and apply the appropriate formula. If the object is irregularly shaped, then the displacement method must be used.
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