Chem 105A Lab Exam - Experiment 1, Density fo Liquids

What is the objective of experiment 1?

To determine the mass % concentration of an unknown NaCl (aq) solution by comparing the density of the unknown to the densities of a series of solutions of known mass % concentration

How did we compare these solutions? What equation did we use?

Measured the mass and volume of the solutions

Compared densities

What tool did we use to measure volume in this lab?

Graduated pipet and a buret

When can graduated cylinders be used?

When imprecise volume measurement is okay

What is random error? What does it affect?

Error that has an equal and arbitrary chance of happening

Affects precision

What is systematic error? What does it affect?

Error that is skewed in one direction and can be attributed to to experimental design, poorly calibrated instruments, or user error.

Affects accuracy

How can we quantify accuracy if we know the true value?

percent error

What is the absolute error in 4.69 +/- 0.04?

0.04

What is the relative error in 4.8 +/- 0.5?

0.5/4.8

What are the two definitions of precision? Which one are we concerned with in experiment 1?

1. number of sig figs an instrument can measure

2. measure of repeatability of results

We are concerned with 2

How can we quantify the precision of results?

Standard deviation and percent standard deviation

How do you calculate percent standard deviation?

(standard deviation) / (average value) x 100%

What were the known solutions we measured in this lab?

2%, 4%, 6%, 8% NaCL and DI water (pooled DI water with four other students)

Why did we measure each solution twice?

To determine if pipets or burets gave more accurate results

What measurements did we need for this lab?

1. Mass of beaker

2. Mass of sample + beaker

3. Mass of sample

4. Initial Pipet/Buret Reading

5. Final Pipet/Buret reading

6. Volume delivered

7. Temperature (water)

How did we dispose of the NaCl?

Poured down the drain and rinsed with water

What was the relationship between percent concentration and density for DI water and the known concentrations?

As % concentration increased, density increased

How did we determine the mass % concentration of the unknown?

y = mx + b

LINEST information

y = density

m = slope

x = % concentration

b = y-intercept

Which method did we find yielded the best results for density?

Buret method

Both more accurate and more precise

What is one possible source of error for experiment 1?

Water leftover in the beaker between trials which could have diluted the next solution

What is one way the density could have been too high?

If the volume in the pipet was underestimated