Chem Lab - Final Exam Questions with 100% accurate solutions (A+ GRADED )

Three methods of separating compounds

Filtration, sublimation, evaporation

Filtration

the process that separates a solid from the liquid in a heterogeneous mixture

Sublimation

A change directly from the solid to the gaseous state without becoming liquid

Evaporation

Removing a liquid from a mixture by turning it into a gas.

Solve for the mass of a chemical in a container

[mass total] - [mass empty dish] = [mass of chemical]

Fire diamond colors

Blue (health risk), Red (flammability), Yellow (reactivity), white (other comments; ie reacts with water).

Hydrates

Compounds that contain water molecules as part of their crystaline structure (ex: MgSO₄ ⋅ 7H₂O)

Lab three was called "hydration of an Epsom salt"... so what's anEpsom salt?

A magnesium sulfate hydrate which can exist in a number of states ranging from [MgSO₄ ⋅ H₂O] to [MgSO₄ ⋅ 9H₂O]

What was the objective of the third lab titled "hydration of an Epsom salt"

To experimentally determine the number of water molecules per formula unit of Epsom salt. We know Epsom salt is [MgSO₄ ⋅ XH₂O] and we're solving for X.

Limiting reagent

any reactant that is used up first in a chemical reaction; it determines the amount of product that can be formed in the reaction

Theoretical yield

the maximum amount of product that can be produced from a given amount of reactant

Actual Yield

the measured amount of a product obtained from a reaction

How do you calculate percent yield?

AY/TY x100%

Solving for limiting reagent

Using molecular weights of the PT, convert each of the reactants into moles. Now utilize the mole-to-mole ratios of each reactant to the end product to convert to the amount which could be synthesized. Whichever reactant produces less can be identified as the limiting reagent.

What does it mean if a chemical is available "in excess" when calculating limiting reactants?

It is not the limiting reactant and can be treated as if an indefinite amount exists.

Explain the procedure in lab four named "Limiting excess reagents"

Mix together the chemicals benzaldehyde and acetone in a test tube. Mix with catylist NaOh/Ethanol, which don't react with the chemicals but expedite their otherwise-slow rate of reaction. The initial chemicals will react to form dibenzalacetone precipitate, a solid, leaving a decent amount of liquid left over. Use a vacuum filtration system to seperate the solid from the liquid. Take the now-seperated solid and heat in oven to remove the last of the liquid.

In lab four, "limiting excess reagents" we synthesize dibenalacetone from benzaldehyde and acteone and then mass the result. What's the importance of this recorded mass?

We compare the result to the mathematically-calculated theoretical yield. From this, we can calculate how effective our reaction was by solving for the percent yield.

What is the objective of lab five, titled "titrations, accuracy and precision"?

Determing the concentration of our NaOH solution through titration.

What is the purpose of a titration?

To use a solution of known concentration to determine the concentration of another solution

What is molarity?

Moles/L

When completing a titration, what term will we use to describe concentration?

Molarity

When measuring precisely from a burret, a curve (meniscus) appears at the top you're trying to measure from. How do you know where to read your measurement from?

Always read from the bottom of a meniscus

How do we find the volume of fluid used when dispensing fluid from a burret?

The volume used is the difference between the final volume and initial volume.

V.used = V.final - V.initial

In lab five, titled "titrations, accuracy and precision" we determine how many liters of one reagent it took to trigger the die effect which stains everything pink. What was the purpose of this?

If we know how many liters (V.used) it took to reach equilibrium, we can calculate the unknown molarity. Since you're provided the number of mols in the unknown fluid, simply plug V.used (in L) into the equation [molarity=mols/L] and solve for the molarity.

What is the procedure for lab six, titled "formula weight of a copper compound?"

We start with a copper compound of unknown composition. We react it with magnesium, which is a lot more electronegative. The magnesium donates electrons to the Cu, displacing it and allowing us to physically seperate it from its compound. Determine mass on a scale.

In lab six, titled "formula weight of a copper compound" we calculate the mass of copper in a unknown copper compound in an attempt to identify and name the compound. Why is this helpful?

We can use grams of copper to calculate the moles of copper since we know the MM of Cu off the periodic table. Then we multiply by the mole-to-mole ratio with our unknown compound (known) to identify the moles of our unknown compound. Now that we know how many moles of the unknown we have, we can compare that to a list of known MM to see which it is.

Qualitative analysis

Observation of physical properties (change of color, formation of a precipitate, evaluation of a gas, change in temperature) to identify its chemical makeup.

Qualitative analysis versus quantatative analysis

'Which substances are present' versus 'how much'

How could we use knowledge of Clorine ions' solubility to conduct a qualitative analysis on a compound?

If we know that a precipitate forms when X interacts with Cl and we mix the two, the absence of a solid would show that the compound doesn't contain Cl.

Lab seven, "the identification of unknown metal ions" instructs us to predict whether or not a combination of chemicals will result in a precipitate. How is this accomplished?

Consult a solubility table. If the chemical is not listed on the soluble column for that compound, a precipiate will form.

(see page 75)

What is the Dumas Method?

a method used to determine the molar mass of a volatile liquid

Percent error formula

%error = (expected-actual)/actual x100%

Experiment eight, "the Dumas Method" involves one experiment involving a balloon. Explain the procedure of that experiment.

Place a balloon in a beaker and fill the beaker with water. Remove the balloon and see how much the water went down. This is the volume of the gas in the balloon. Try this three more times at different temperatures to understand the effects of temperature on volume. [As the temperature doubles, volume doubles]

Experiment eight, "the Dumas Method" involves one experiment in which the mass of a gas is calculated experimentally in an effort to calculate molar mass. Why would we want to do this?

Since we can calculate the moles of gas using the known P, V and T, all we need is the experimental mass to plug into MW=(mass × R × T)÷(P × V)

Now we know the molar mass and can compare that value to a table of known gases.

Experiment eight, "the Dumas Method" involves one experiment involving the vaporization of a gas. Explain the procedure of that experiment.

First you calculate the number of moles of a gas by determining P, V and T. This experiment involves vaporizing an unknown liquid to push the air out of the flask, allowing the unknown to recondense and massing it.

In which experiment is Hess' law important?

The calorimetry experiment, #9

Conservation of heat law

If something loses heat, something must gain that heat

Accuracy versus precision

'close to reality' versus 'consistent'

What must be done in the calorimetry experiment before the neutralization reactions can be accurately measured?

Calibration of the two thermometers to match one anothers' results. Then calculate how much heat (per unit) is lost to the calorimeter.

During which of the nine experiments did we utilize computerized charts to track changes over time?

Lab nine, "Calorimetry and Hess' law"

In lab nine, "Calorimetry and Hess' law," what is the meaning of T₀ on our graph for temperature of the two compounds?

The temperature the instant the two compounds are mixed.

Relevant to Lab nine, "Calorimetry and Hess' law" what occurs temperature-wise when an acid and base at room temperature are combined?

The acid-base mixture will absorb heat, essentially becoming warmer as they react.

What is the goal of experiment 10, "the synthesis of alum?"

Convert aluminum metal into something called "alum" and calculate the % yield.

What is the importance of having a high % yield?

Working in an industrial plant, even a 0.1% improvement in percent yield can result in massive profits when you're producing 160,000,000 tons a year as a nation. (true fact)

Explain the procedure of experiment 10, the "synthesis of Alum"

Tear a strip of aluminum into pieces. Add KOH to the foil and allow a reaction to occur. Add H₂SO₄ and wait. Crystals will form. Rinse crystals with ethanol and put them in the oven. Now the crystal product is dry and ready to be massed. We can compare this mass with the TY to see how efficient our procedure was. Expect ~50% efficiency.