CHEM 121 LAB FINAL EXAM

% Recovery

((Mass of Mixture Component 1 + Mass of Mixture Component 2 + Mass of Mixture Component 3)/ Starting Mass of Mixture) * 100

% Composition

(Mass of Mixture Component / Total Mass of Mixture) * 100%

Calculate the % composition of each component in the original mixture.

(PL Question #2 - Exp 2)

In a precipitation reaction, what symbol identifies the precipitate product?

(s)

A cube of plastic 1.3 × 10−5 km on a side has a mass of 2.1 g. What is its density in g/cm3?

0.96 g/cm^3 (PL Question #1 - Exp 1)

What visible signs indicate a precipitation reaction when two solutions are mixed?

1) A hazy or cloudy appearance 2) Solid settling out in the test tube

In order to use a pipet, place a (1) at the top of the pipet. Use this object to fill the pipet such that the (2) of the liquid is even with the volume line. Release the liquid, touching the tip of the pipet to the side of the container if necessary to release the last drop (3) the pipet tip.

1) Bulb or pump 2) Meniscus 3) Outside

Identify effective techniques for accurate pipet use.

1) Do not let liquid enter the pipet bulb or pump. 2) Measure liquid by aligning the meniscus with the volume line.

Identify the best heat source for each situation listed: 1) A test tube must be heated to a very high temperature. 2) A flask or beaker must be heated to a very high temperature. 3) A test tube needs to be warmed gently to a constant temperature.

1) Place in a Bunsen burner flame. 2) Place on a hot plate. 3) Clamp in place in a water bath on a hot plate.

When using the Tare function on a balance, start by (1). Then (2) to cancel out that mass. Finally, (3) and record the mass of your sample.

1) Placing the glassware on the balance 2) Press the Tare/Zero button 3) Add your sample to the glassware

After assembling a gravity filtration apparatus, begin the separation of a mixture by (1) it into the filter paper. Pour the entire contents of the mixture into the filter paper, (2) the solid in the filter paper if needed to speed up the filtration. After all the liquid has drained through the filter paper, finish by rinsing the solid with (3).

1) decanting 2) stirring 3) solvent

After a mixture has been poured through filter paper, the (1) component remains on top while the (2) flows through. To collect the solid, spread the filter paper (3) and allow the solvent to (4).

1) solid 2) liquid 3) on a watch glass 4) evaporate

Consider the process of separating a mixture into its components. The added masses of the recovered components will (1) equal 100% of the starting sample mass. Some of the mixture may have been lost during separation, leading to a percent recovery of (2). Some of the mixture components may contain water, leading to a (3) total mass of recovered sample and a percent recovery of (4).

1) sometimes 2) less than 100% 3) higher 4) more than 100%

When heating a solution to boiling on a hot plate, start by (1). Then, turn the heat to (2) to start. If necessary, (3) after waiting about ten minutes without seeing boiling.

1) starting and stabilizing the stir function 2) a medium heat setting 3) turn up the heat setting

A 9.55-g sample of a solid was placed in a 15.00-mL flask. The remaining volume was filled with benzene, in which the solid is insoluble. The solid and benzene together weigh 19.33 g. The density of the benzene is 0.876 g/mL. What is the density of the solid?

2.49 g/mL (PL Question #2 - Exp 1)

Suppose a student starts with 2.4394 g of a sand mixture and separates the components into 1.3012 g of NaCl, 0.5210 g ofSiO2, and 0.4503 g ofCaCO3. Based on the amount of recovered, what is the percent ofSiO2 in the starting mixture?

21.36%

A 24.5 g sample of solution has a density of 0.768 g/mL. What is the sample volume (in mL)?

31.9 mL v = mass/density v = 31.9 mL

A series of measurements in the lab led to an experimental result of 32.9 mL, with a calculated standard deviation of 0.3 mL. What is the standard way to report this result?

32.9 ± 0.3 mL

How many significant figures are in the number 510.9?

4

A pill that is 91.5% aspirin is found to contain 50.93 mg of aspirin. What is the mass of the pill in milligrams?

55.7 mg

4.0000 g

A balance with ±0.0001 g tolerance

1.230 g

A balance with ±0.001 g tolerance

5.43 g

A balance with ±0.01 g tolerance

You analyzed 2.475 g of a mixture of NaCl, SiO2, and CaCO3. You isolate 0.825 g NaCl, 0.825 g of CaCO3, and 0.580 g SiO2. You were told that the original mixture contained 35% NaCl, 45% CaCO3, and 20% SiO2. Report two possible sources of error.

According to the data provided, we should have recovered approximately 0.866 g of NaCl, 1.114 g of CaCO3, and 0.495 g of SiO2. One possible reason for the source of error is that we could have reacted the sample with a Hydrogen Chloride (HCI) before extraction, resulting in a difference in composition. Another possible reason for the source of error is that another chemical could be being formed after the reaction between CaCO3 and HCI, resulting in decreased amounts of CaCO3 and SiO2.

What occurs when aqueous silver nitrate, AgNO3, reacts with aqueous potassium sulfate, K2SO4?

Ag2SO4 forms as a precipitate.

What is the best course of action if a gravity filtration setup seems unsteady?

Clamp the setup to a ring stand.

What is the goal of qualitative analysis?

Determining the composition of a substance

What is the advantage of fluted filter paper shown compared with cone-shaped filter paper? (The filter paper is fluted)

Fluted filter paper has more surface area suitable for collecting solid.

Standard Deviation Formula

Page 26 in Notebook

Sodium chloride (NaCl) was isolated from SiO2 and CaCO3 by dissolving it in distilled water. Unmentioned is that CaCO3 is slightly soluble in water (0.001 g/100 mL). Describe the effect this fact will have on each component of the mixture: will the "isolated" mass of each be greater than, equal to, or less than that in the sample?

Since CaCO3 is slightly soluble in water, some of the CaCO3 will dissolve upon the extraction of NaCl. The resulting solution after extraction (which contains a small amount of CaCO3) will be then be separated with the NaCl once the solution is evaporated. Thus, the isolated mass of NaCl should be greater than the amount in the sample. Since some of the CaCO3 will be dissolved in water, the isolated mass of CaCO3 will be less than the amount in the sample. The mass of SiO2 will equal the amount in the sample because the method of separation for SiO2 is undisturbed by the solubility of CaCO3 in water.

Which states of matter can be separated by gravity filtration?

Solid and liquid

What statistical calculation, which results in the same units as the measured data, represents the average variance of data points from the calculated average of the data set? Proper spelling is required.

Standard Deviation

Precision

The closeness among a set of measurements.

Accuracy

The closeness between a measurement and its accepted value.

What is the purpose of stirring a solution while it is being heated?

To help evenly distribute heat throughout the solution

Could the separation in this experiment have been done in a different order? That is, isolation of SiO2 first, followed by separation of CaCO3 and NaCl. Explain your answer.

Yes, the experiment could have been done this way. First, we could mix the sample of SiO2, CaCO3, and NaCl with water. After stirring the sample with water, we could separate the SiO2 through decantation. After pouring the supernatant liquid into a separate beaker, the insoluble CaCO3 will settle at the bottom of the beaker. This means that we can separate NaCl from CaCO3 through decantation again. Lastly, evaporation can be used to separate the NaCl that has been dissolved in water.

A graduated cylinder contains 10.00 mL water. A 14.74 g piece of aluminum is added to the water, and the volume rises to 15.46 mL. What is the density of the aluminum in g/mL?

density = mass/volume d = 14.74g / 5.46mL = 2.70 mL