Density of Liquids and Solids Experiment 2

What is the Density? Density is how compact a substance is He atoms have a mass of 4 amu and are about 3 nm apart Density is how compact a substance is How close are all the atoms to each other and how heavy are those atoms? We define the density, d, to be 𝒅= 𝒎𝒂𝒔𝒔 𝒗𝒐𝒍𝒖𝒎𝒆 For He 𝑑 𝐻𝑒 = 4 𝑔𝑟𝑎𝑚 22.4 𝐿 =0.00004464 𝑔 𝑚𝐿 For Au 𝑑 𝐻𝑒 =19.3 𝑔 𝑚𝐿 Gold is 0.5 million times more dense! Au atoms have a mass of nearly 197 amu and are about 0.4 nm apart

Relative Density In liquids and gases the densest material falls to the bottom and the least dense materials float to the top because the force of gravity is stronger on more dense objects Least dense S1 – Cork Mineral Oil – L1 S2 – Ice S3 – Rubber Stopper Corn Syrup – L2 S4 – Marble (Glass) Most dense Glass > Corn Syrup > Rubber > Water> Ice> Mineral Oil> Cork

Measuring the Density To measure the density we need to measure the mass, m and the volume V Then we divide the mass, m, by the volume, V, to get the density d 𝒅= 𝒎𝒂𝒔𝒔 𝒗𝒐𝒍𝒖𝒎𝒆 = 𝒎 𝑽

Density: Getting the Mass m When dealing with liquids and gases, we measure the mass of our unknown, munk, indirectly by a technique called weighing by difference. First weigh empty flask or beaker to get m1 Then weigh the same flask with some amount of your unknown in it to get m2 The mass of your unknown munk = m2 – m1 = 204.3242g - 101.0120g munk = 103.3122g m1 = 101.0120 g m2 = 204.3242 g

Density: Getting the Volume: Solids Then to get the density we need to measure the volume the way this is measured is different depending on if we have a solid or a liquid We can get the volume Vunk of an insoluble, irregularly-shaped, solid using volume by displacement Vunk = V2 – V1 = 266 mL – 195 mL = 71mL V2 = 266 mL V1 = 195 mL

Density: Getting the Volume: Liquid Best to use a pipette Or burette

Volume of Rectangular Solid Measure, length L, width W, and height H

Calculating the Density Density of a Liquid 10.00mL of a liquid pipetted into a flask, the mass of the liquid (weighing by difference) 10.1410g 𝑑= 𝑚 𝑉 = 10.1410 𝑔 10.00 𝑚𝐿 =1.0141 𝑔 𝑚𝐿 =1.014 𝑔 𝑚𝐿 Density of a Rubber Stopper Stopper mass = 8.453g, displaces 6.5mL of water in a graduated cylinder 4 sig. figs 2 sig. figs

Thickness of Aluminum Foil Aluminum foil is too thin to measure its thickness, H, with a ruler Instead we measure the thickness by using the density of aluminum to help us Measure the dimensions of a rectangular piece of foil W = 10.75 cm L = 10.10 cm Measure the mass of the foil = M M = 0.4500g Knowing the density of Aluminum = 2.70 g/cm3 2.70 𝑔 𝑐𝑚 3 = 𝑀 𝑉 = 𝑀 𝐿×𝑊×𝑯 𝑯= 0.4500𝑔 10.10𝑐𝑚×10.75𝑐𝑚×2.70 𝑔 𝑐𝑚 3 𝑯= 𝑀 𝐿×𝑊×2.70 𝑔 𝑐𝑚 3 𝑯=0.00154𝑐𝑚=1.54× 10 −3 𝑐𝑚

Objectives To observe the relative densities of some common liquids and solids To determine the densities of water, an unknown liquid and an unknown rectangular solid To determine the thickness of a piece of aluminum foil using the density concept To gain proficiency in pipetting, weighing by difference and determining a volume by displacement

Tonight Measure density of water with 10mL pipette and 125 mL Erlenmeyer Flask (2 Trials) Measure density of an unknown liquid with 10mL pipette and 125 mL Erlenmeyer Flask (2 Trials) Density of a #2 Rubber Stopper – (water displacement with 100mL Cylinder – 2 Trials) Density of rectangular solid – block of wood Thickness of a piece of rectangular Aluminum foil by measuring its mass and assuming it has a density of 2.70 g/cm3