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Lab 3: determination of density

Find the density of an unknown liquid and an unknown solid, then identify them. The forms hold your numbers and check them; the arithmetic and the sig figs are yours.

Review of the graded report and Quiz 3.

What the lab asks for

density (g/mL) = mass (g) ÷ volume (mL)

Liquid: glassware

  • 250 mL beaker marked "Waste"
  • 150 mL beaker as the reservoir, marked in pencil with your liquid's code letter (A, B or C)
  • 100 mL beaker for the density determination
  • 10 mL volumetric pipette

Liquid: procedure

  1. Pour about 50 mL of the unknown into the marked 150 mL beaker.
  2. Weigh the clean, dry, empty 100 mL beaker and record that weight in every trial column.
  3. Fill the 10 mL volumetric pipette and add the liquid to the weighed beaker.
  4. Weigh the beaker with the liquid in it and record the weight.
  5. Discard the liquid into the waste beaker, not back into the reservoir.
  6. Dry the beaker with a paper towel and repeat, for three weighings in all.

Solid: glassware

  • The same 250 mL waste beaker
  • 25 or 50 mL graduated cylinder, the 50 mL one for the larger round samples
  • 3 weigh boats, marked 1, 2 and 3
  • A vial of unknown solid from the bench
  • 150 mL beaker with about 100 mL of distilled water

Solid: procedure

  1. Weigh each of the three weigh boats and record the weights.
  2. Put roughly a third of the solid into each boat. For the large round beads, 2 or 3 per boat.
  3. Weigh each boat again with the solid in it, and take the difference for the mass of solid.
  4. Fill the graduated cylinder about half full with distilled water and read the level carefully.
  5. Tilt the cylinder, pour in the solid from boat 1 without splashing any water out, and read the level again.
  6. Empty the water and solid into the waste beaker, then repeat with boats 2 and 3.

From class: use the 25 mL graduated cylinder for the solids, not the 10 mL. Weigh the bowl once and reuse that value across the trials rather than taring each time; tare just means zeroing the balance. Put the solids back in their original jars, dried, for the 6 o'clock class. This lab needs no formal report, but Lab 4, hydrates, does. Our group's liquid came out near 1.28 g/mL against 1.326 for dichloromethane.

Source Posted CHM 101 Lab 3 notes on Brightspace

Data form 1: density of the unknown liquid

The mass of the 100 mL beaker goes in every trial column, so it is one box here. Each trial is one 10.00 mL delivery from the volumetric pipette. The blue boxes are the ones you calculate.

Unknown liquid densities, from the lab notes (g/mL)

Table 3.1 lists five, and your unknown is meant to be one of them. Ethanol is added here because 0.789 g/mL sits 0.003 from methanol's 0.792, close enough that a measurement near 0.79 cannot separate the two. The pre-lab key puts water at 0.9970 g/mL at 25 °C, from Table 2.2 in Lab 2; Table 3.1 rounds it to 1.000.

Data form 2: density of the unknown solid

Weigh boat first, then the boat with the solid in it, then the water level before and after the solid goes into the graduated cylinder. The blue boxes are the ones you calculate.

Unknown solid densities, from the lab notes (g/cm³)

Your unknown is one of these seven.

Your numbers

Every box saves as you type, on this device

Practice Unit Tracks: conversions and sig figs Calculator Measurement and units

The Check buttons are strict about sig figs. A subtraction keeps decimal places, a division keeps significant figures, and an average keeps the significant figures of the trials: 8.6, 6.7 and 13 average to 9.4, not 9. Percent error is the difference from the published value, divided by that value, times 100, and it follows the same chain: the subtraction fixes the decimal places, then the division fixes the sig figs. A standard deviation counts either written to the same decimal place as your average or to two significant figures, and the sample and population versions are both accepted.

Post-lab quiz

Questions on the method and the calculations, written from the posted lab notes: the official Lab 3 quiz key on Brightspace stays locked until October 7. Each question says only whether your answer is right, and after a miss you can show it.