Chemistry Lab I Review Questions

What advantages does using a pH probe have over using just an indicator for titrations?

A lot more simple than test strips or titrating and does not use color.

How could you separate zinc chloride from SiO2?

Simple add water. Zinc chloride is soluble in water while the SiO2 is not. Then, you could filter the mixture to get pure SiO2.

Define the process of sublimation.

Sublimation is the conversion between the solid and the gaseous phase of matter with no intermediate liquid phase. Sublimation is an endothermic process that occurs at temperatures and pressures below a substance's triple point.

Why is the NaCl extracted three times as opposed to only once?

The NaCl is extracted three times in order to improve the accuracy of the experiment when it comes to calculating the percent mass.

A student fills a 100 milliliter beaker with enough water to go to the 30 milliliter mark. They weigh the beaker and the mass is 29.54 grams. What is the actual volume in the beaker? Using the density of water at 25 degrees celsius.

The actual volume in the beaker is 1.02 g/mL.

How does experiment number two illustrate the conservation of matter?

The conservation of matter states that matter can not be created or destroyed, just simply transformed. Therefore, when transforming the mixture given, we simply heated, cooled, and filtered the mixture to get the desired result. Any liquid that was heated would result in vapor in order to leave a dry substance. Or, if there was liquid with no heat applied, the filtration simply suctioned off the excess liquid. When cooled, the mixture simply separated, but did not create two new substances. Nothing new was made nor was anything destroyed; the experiment simply employed the phases of matter to extract a substance that was within the mixture.

Explain what effect each of the following will have on the actual yield of the [Ni(C2H8N2)3]Cl2. a.) Student used 10 mL of water to dissolve the NiCl2 • 6H2O. b.) Student used water to rinse the [Ni(C2H8N2)3]Cl2 product. c.) Student used 12.0 mL of C2H8N2 but recorded the volume as 8.0 mL and used en as the limiting reagent in the calculation.

a.) The actual yield will decrease because NiCL2 • 6H2O is sensitive to large amounts of water. b.) Actual yield will decrease because some of the product will dissolve. c.) Actual yield will decrease because there will be less moles which will decrease the moles of en.

a.) What was done during filtration to help reduce errors that would lead to a percent yield greater than 100%? Explain how this reduced the error. b.) What was done during filtration to help reduce errors that would lead to a percent yield less than 100%? Explain how this reduced the error.

a.) Using a tool to scrape just the solid onto the watch glass because if anything other than the solid was weighed then the actual yield would be inaccurate. b.) Completely filtering the solid until it is dry because the added moisture would affect the actual yield, and therefore the percent yield would be higher.

Classify the following as either a pure substance or a mixture; if it is a mixture, state whether it is heterogenous or homogeneous. a.) concrete b.) tomato juice c.) marble d.) seawater e.) iron

Concrete: heterogenous mixture Tomato juice: heterogenous mixture Marble: heterogenous mixture Seawater: heterogenous mixture Iron: homogenous mixture

Why should you never place a hot object on the pan of the balance?

Convection currents tend to buoy the object and lead to an inaccurately low mass. Also, the heat can damage the balance which could affect the mass reading as well.

What is meant by the term decantation?

Decantation means to separate a mixture by pouring the liquid off of the solid.

Red and green light have a wavelength of about 650 nanometers and 490 nanometers, respectively. Which light has the higher frequency? Which has the higher energy?

Green has the higher frequency and therefore has the higher energy.

If boron emits radiation at 518 nanometers, what color will boron impart to a flame?

Green.

What two substances are flammable in experiment four?

Hydrogen gas and ethanol.

What will you observe after the reaction is complete if C2H8N2 is present in excess?

If C2H8N2 is in excess, the solution will be basic, and the color would be pink.

Nick, Ethyl, and Amy reacted 3.60 g of NiCl2 • 6H2O with 8.50 mL of 25% C2H8N2 and obtained 2.83 g of [Ni(C2H8N2)3]Cl2. The filtrate was dark blue. a.) What is the limiting reactant? b.) What is their theoretical yield? c.) What is their actual yield? d.) What is their percent yield?

a.) C2H8N2 b.) 3.51 grams c.) 2.83 d.) 80.6%

How many milliliters of 0.250 M NaOH is required to neutralize 30.4 mL of 0.146 M HCl?

- Concentration of HCl(M1)=0.146 M - Volume of HCl V1=30.4mL - Concentration of NaOHM2=0.250M - Volume of NaOH V2=? - M1V1=M2V2 - V2=M1V1/M2 = 0.146M*30.4mL /0.250M = 18.4832mL - The Required volume V2=17.7536mL

Name six methods of separating materials.

1. Distillation 2. Sublimation 3. Filtration 4. Decantation 5. Chromatography 6. Extraction

Define a Bronsted-Lowry acid and base.

A Brønsted-Lowry acid is any substance (molecule or ion) that can donate a hydrogen ion (H+). A Brønsted-Lowry base is any species that can accept a hydrogen ion (H+).

What distinguishes a mixture from an impure substance?

A mixture is two or more pure substances. An impure substance is one substance contaminated by other small amounts of substances.

What is the difference between absorbance and transmittance?

Absorbance is the light that is taken in my the object and the light does not pass through. Transmittance is the light that passes all the way through the object.

Describe the process of absorption and emission.

Absorption happens when an excited electron jumps energy levels. Emission happens when an electron in a higher excited state drops to a lower one which produces light.

How is accuracy and precision determined?

Accuracy is determined by deciding how close a set of data is to the standard value. Precision is determined by how close the data is to each other independent of the standard value.

Define accuracy and precision.

Accuracy refers to how close a measured value to a standard or known value is. Precision is how close values are to each other regardless of what the standard value is and describes how repeatable something is.

How would the absorbance be affected if the lid on the spectrometer is not closed properly while taking the measurement?

Ambient light from the room would leak into the detector and create an inaccurate reading.

How is a Bronsted-Lowry acid/base different from the Arrhenius definition?

An Arrhenius base increases the concentration of OH- ions. A Brønsted-Lowry acid is any species that donates a proton to another molecule. A Brønsted-Lowry base is any species that accepts a proton from another molecule.

What are equivalence points and end points? How do they differ?

An equivalence point is a point in titration at which the amount of titrant added is just enough to completely neutralize the analyte solution. An end point is the point during a titration when an indicator shows that the amount of reactant necessary for a complete reaction has been added to a solution. The equivalence point is also a point where the chemical reaction comes to an end while the endpoint is the point where the color change occurs in a system.

Given an example of redox and metathesis reactions.

An example of redox is photosynthesis. AN example of metathesis is the double displacement reaction between sodium chloride and silver nitrate to form sodium nitrate and silver chloride.

Explain Beer's Law.

Beer's Law states that the light absorbance of a solution is directly proportional to the concentration of the attenuating species and the optical path length.

Suggest a way to determine whether a colorless liquid is pour water or a salt solution without tasting it.

Calculate the density of the substance since salt typically makes a solution more dense.

What are the advantages of using just an indicator to perform titrations?

Can measure pH at low frequency.

Why does the density of water change with temperature? Is this unique to water?

Density changes with temperature because the volume changes with the temperature. With heat, the volume typically increases as molecules move apart. With cold, the volume will decrease. As volume increases, density will decrease. This is unique to water.

What is meant by the term filtration?

Filtration means to filter the mixture by passing the mixture through a semi-permeable paper to extract the liquid from the solid.

Define gravimetric analysis.

Gravimetric analysis describes a set of methods used for the quantitative determination of an analyte based on its mass.

Describe the photoelectric effect.

Removal of an electron by the total absorption of an x-ray.

What is the Balmer series?

The Balmer series is one of a set of six named series describing the spectral line emissions of the hydrogen atom.

What is the wavelength of light that has a frequency of 76 Hz?

The wavelength is 3944637.61 meters.

What would happen if not all of the oxygen is evaporated from the sample? How would this effect the results of the experiment?

If not all of the oxygen is evaporated, the experiment would not perform accurately. Then, when the calculations are being performed, the experiment's percent error would be inaccurate.

What will you observe after the reaction is complete if NiCl2 • 6H2O is present in excess?

If the NiCl2 • 6H2O is in excess, the solution will be acidic and the color would be blue.

How do decantation and filtration differ? Which should be faster?

In filtration, the liquid is filtered by being passed through a semi-permeable substance (filter). In decantation, the liquid is physically separated from the liquid through pouring. Decantation tends to be faster since no other substances are involved.

Was the color of the [Ni(C2H8N2)3]Cl2 product the same when NiCl2 • 6H2O was the limiting reactant as compared when C2H8N2 was the limiting reactant?

No. The color for NiCl2 • 6H2O dark blue, and the color for C2H8N2 is pale pink.

A student determined the following actual volumes of 35 milliliters of water from a 100 milliliter graduated cylinder: 34.2 mL, 34.6 mL, 35.9 mL, and 36.1 mL. What is the percent error and standard deviation of those results?

Percent error for 34.2 mL: 2.3% Percent error for 34.6 mL: 1.1% Percent error for 35.9 mL: 2.6% Percent error for 36.1 mL: 3.1% Standard deviation: 0.94

Define percent yield in general terms?

Percent yield = (actual yield/theoretical yield)(100%).

When will the reactions proceed to completion?

Reactions will proceed to completion whenever one of the products is physically removed from the reaction medium. This often occurs when a gas or solid is formed.

Name the colors of visible light beginning with that of the lowest energy (longest wavelength).

Red, orange, yellow, green, blue, and violet.

How would the value of density for water be affected if the mass of the water was recorded at 5.71 grams instead of 5.17 grams for a volume of 5.22 milliliters? Will it be higher, lower, or no effect?

The density would be at 1.09 g/mL at 5.71 grams rather than 0.99 g/mL at 5.17 grams. Therefore, the density would increase.

What is parallax? Why should you avoid it?

The effect of failure to level eyes with the bottom of meniscus; avoid it because it creates inaccurate readings and results.

What is the energy of a photon with a wavelength of 680 nanometers?

The energy is 2.92 x 10^-19 J.

Define limiting reactant, excess reactant, and aliquot.

The limiting reactant is the reactant which is completely consumed during the reaction. The excess reacting is a reactant present in an amount in excess of that required to combine with all of the limiting reactant. An aliquot is an exact portion of a sample or total amount of a liquid.

If 3.35 g of Cu(NO3)2 are obtained from allowing 2.25 g of Cu to react with excess HNO3, what is the percent yield of the reaction?

The percent yield is 148.9% since the actual yield was larger than the theoretical yield.

What two substances in this experiment are oxidizers?

The potassium chlorate and nitric acid.

Define qualitative analysis.

The process of determining the identities of unknown substances.

How would the results be affected if the solutions used for calibration are not made properly?

The readings for the solutions would not be correct for the experiment.

What would happen if the spectrometer is not blanked properly?

The readings will not be accurate.

A student heats a sample 1.50 grams of sodium chlorate (NaClO3). After two heating and cooling cycles, the sample is reweighed and found to be 1.67 grams. What is the theoretical mass percent, actual mass percent, and the percent error?

The theoretical mass percent is 47.3%. The actual mass percent is 52.7%. The percent error is 11.3%.

Using the Rydberg equation, calculate the wavelength for all the transitions of the Balmer series.

The wavelengths include 410.2 (violet) nm, 434.1 nm (blue-violet), 486.1 nm (cyan), and 656.3 (red) nm.

A mixture was found to contain 1.05 grams of SiO2, 0.69 grams of cellulose, and 1.82 grams of calcium carbonate. What percentage of calcium carbonate is in the mixture?

There is 51.1% of calcium carbonate in the mixture.

Define the term titration.

Titration is the technique and procedure of accurately measuring the volume of a solution that is required to react with a known amount of another reagent.

How are absorbance and percent transmittance related mathematically?

Transmittance is the inverse of absorbance.