CAPP CHEM Lab Questions
E7Q2 Solubility
maximum amount of solute that can be dissolved in 100g of solvent at a given temp.
E8Q2 (methods) Evaportation
suitable to seperate a solid from a liquid solution is heated, and then liquid evaporates and leaves solid.
E1Q6 Why is it important to always use the same balance during the course of an experiment?
The different balances could be off and calibrated differently, which could give you inconsistent masses throughout the experiment, which may give you incorrect results in the end.
E7Q3 normal boiling point
the temp at which vapor escaping from the surface of the liquid has a pressure equal to the pressure present above the liquid at 1atm under normal conditions
E2Q1 Pipets used for the transfer of aqueous solutions are always rinsed with a small portion of the solution to be used before the sample is taken. Calculate the percentage error arising in an experiment when 1ml, 5ml, and 10ml pipets are used for transfer and each pipet contains 5 drops of water adhering to the inside of the barrel. A single drop of water has an apporximate volume of .005 ml
1 ml------- (abs) 1ml-.25ml/1ml * 100 5ml------- (abs) 5ml-5.25ml/5ml *100 10ml------ (abs) 10ml-10.25ml/10ml *100
E3Q4 An alternative procedure for determining the density of a liquid is to pipet a sample of the liquid into a weighted flask and then reweigh the flask to determine the mass of liquid transfered. Would this alternative procedure be likely to give greater precision in the density determination? Why?
Yes using a pipet would give greater precision because a pipet can transfer liquid more accurately due to the pipet being able to measure to a smaller number, as opposed to the flask being more vauge.
E2Q2 It is important to make certain that there is no air bubble in the tip of the buret below the stopcock before the initial reading of the liquid level in the buret is taken. If a .5ml air bublle is present in the tip of a buret, what percent error in 10ml, 20ml, and 40ml samples will result if the air bubble is dislodged during the dispensing of the samples?
10ml------ (abs) 10ml-10.5ml/10ml *100 20ml------ (abs) 20ml-20.5ml/20ml *100 40ml-------(abs) 40ml-40.5ml/40ml *100
E2Q4 What is a meniscus? How do we properly read the liquid level when dealing with a liquid that forms a meniscus?
A meniscus is the curved surface caused by an interaction between the water molecules and the molecules of the surface of the glass container wall. To read the liquid level, read it at eye level, and read the level at the bottom of the curved surface.
E5Q2 Will the presence of an impurity in a sample cause the melting point to increase or decrease? Explain
If there is the presence of impurity in a sample, the melting point will decrease since the substance is no longer pure. Adding impurities to samples will cause the melting points to decrease. An example of this would be when salt is added to icy roads. The melting point of the ice then decreases, making the ice go away.
E3Q5 Your density of sodium chloride solutions should have produced a straight line when plotted. How could this plot be used to determine the density of any concentration of sodium chloride solution?
If you had an accurately plotted graph, you could plug in any concentration value to get the density, and vise versa because the relationship is linear, and directly proportional.
E4Q3 What does it mean to say that the normal boiling point of a pure liquid substance is a characteristic property of the liquid substance?
It means that if the normal boiling point of a pure liquid substance is found, then that substance can be identified. For example if, from doing tests/experiments, you find that the boiling point is 50.5 C, the its possible to determine that the substance is acetone because the boiling point is a characteristic property, which means that under same lab conditions, a given substance will always have that same boiling point.
E6Q8 you were warned not to keep the test tube in the boiling water for very long, to avoid evaportation of the water from the solution. What errror would have been introduced into your determination of solubility if some of the water had evaporated from the solution? would the apparent solubility have been too high or too low? Explain
The error would be that the evaportated water would lead to error with the amoiunt of water that was actually used to dissolve the salt. Whatever solubilitiy value was collected woul dhave been too low. This is because the volume would be lowers, causing the slat to not dissolve as much. There would be more crystals possibly. less salt will evaporate. You have then less solvent.
E6Q1 Saturated solution
a solution which does not disolve any more solute. When a saturated solution is placed in contact with additional solute, the solute neither dissolves nor is deposited from a saturated solution. You have an equalibrium at a certain temp, some of the solute is goig to the solvent, some of solvent is entering the solute.
E7Q8 In this experiment, you qualitatively determined whether or not a give solute would dissolve in each of the three solvents. Suggest a method by which the solubility of a solute in a solvent might be measured on a quantitative basis.
you can take approx 5g of solute, and weigh it to the nearest mg. Then you can add 3.00 plus or minus .01ml of solvent. If it does not completely dissolve, add more solvent in 1.00 plus or minus .01 mg incriments until it is just barely dissolved. Record this amount and do solute divided by solvent.
E5Q5 Why is a thiele tube preferred for melting point determinations, rather than an open beaker of oil?
An open beaker of oil is much more dangerous because it can get extremely hot. If you have an open beaker of hot oil, it is more likely to have errors. If you are using an arm with a thiele tub, it has to keep it at a uniform temp which makes it more accurate and safe.
E2Q3 Why is a rubber safety bulb always used when working with a pipet?
Because it creates suction without using your mouth to draw up liquid, which was a practice previously used but is now considered unsafe.
E1Q2 Explain the following: Weigh approx 5grams of NaCl to the nearest milligram
If instructions say "blah blah blah", it means that you should obtain .450 to .550 grams of the substance. You should still record the actual amount. Unless a procedure explicitly says to get an exact amount, it is a waste of time to get 5.00 g (in this case)
E6Q5 What does it mean to say that a saturated solution is in equilibrium with undissolved solute at a given temp?
It means that its the solution in which no more salt is able to be dissolved.
E2Q6 On the basis of your experience in this experiment, briefly discuss the relative precision permitted by a graduated cylinder, a pipet, and a buret. Give several circumstances under which you would use each instrument, in preference to the other two.
Pipets were the least precise in this experiment, with a difference of .243 ml. The next most accurate would be the buret, with a difference of .210 between actual and calculated volumes. Our most accurate between the three was the graduated cylinder, which had a difference of .190 ml. You would use a pipet to transfer smaller amounts of a substance. A circumstance in which you would use a graduate cylinder is if you need a very specific amount because it is possible to add or remove single drops. A buret would be used when you want to get a single drop at a time.
E7Q5 A common mistake many students make is to confuse wheter or not a solute is soluble in a solvent with how quickly a solute dissolves. What factors affect the solubility of a solute in a solvent? What factors affect the rate at which a solute dissolves in a solvent?
Some factors that affect solubility include the nature of the solvent and, if the solvent is a gas, pressure. The nature of the solvent affects solubility becasue different substances have different characteristics, and would have a different amount of solute that can dissolve depending on what type of solute it is. Temperature doesn't always affect solubility, but if it is soluble in the solvent that it is in, it will.
E3Q1 A term that is easily confused with density is specific gravity. For example, urinalysis reports commonly give the specific gravity of the sample rather that the density. What is specific gravity? What are the units of specific gravity?
Specific gravity is the ratio of the density of a substance to the density of a standard, usually for a liquid or solid, and air for gas. Specific gravity is a ratio of two densities, so it has no units. Ratio of the density of the substance to the density of the reference substance, which is usually water at the same temp.
E2Q5 Precision volumetric glassware may be marked with the legends "TC" or "TD" along with temperature? whats that mean?
TD means to deliver, meaning that the pipet or buret will deliver that certain amount. TC means to contain, meaning that the pipet or buret will contain that certain/specified amount.
E1Q8 Why should the reagent chemicals never be weighed directly on the pan of the balance?
The chemicals could spill onto the balance, causing damage to the electronics within the balance. Also, if it is not completely cleaned up, it can add mass to your object.
E1Q3 The force of gravity on the moon is approx 1/6 than on the earth. If an object that has a mass of 1kg on the earth were sent to the moon, would its mass change? would its weight be different? why or why not?
The mass would not change because mass is not affected by the foce of gravity, it is measured only by the amount of matter in substances. Its weight woul dbe different because the gravitational force has changed from 9.8 m/s (like on earth) to 1/6 of that on the moon. So it's weight would be 1/6 of what it is on earth.
E7Q7 Why does a pure substance always have the same melting and boiling points at a given barometric pressure?
The melting and boiling points are characteristic properties, so they do not change from sample to sample under the same conditions If it is a pure substance, it has the same compostition and all of the particles will react in the same ways.
E1Q7 What error is introduced in a mass determination if the object being weighed is warm?
The object should be at room temp because warm air rises, whicih can cause motion within the air around the object which can then affect the mass by giving it a lower mass than the object really has.
E3Q2 What error would be introduced into the determination of the density of the regularly shaped solid if the solid were hollow? Would the density be too high or too low?
The volume would be higher due to the solid being hollower because of the air inside the hollow object. This would cause the density to be lower because the volume changed.
E4Q5 Food products such as cake mixes often list special directions for cooking the products in high altitude areas. Why are special directions needed for such situations? would a food take a long or shorter time period to cook.
There are special directions because of the different air pressures at different altitudes. For example, the air is thinner and has less pressure, so the time it takes for a substance cooking would be lower since there is less pressure, making the temp rise.
E8Q5 Some mixtures of volatile liquids cannot be completely seperated by fractional distillation because of the formation of what are called azeotropes among the liquids. Definition of azeotropes
They are a mixture of at least two different liquids. Their mixture can have differing melting points from the components.
E6Q4 In the introduction to this experiment it states that "the solubility of a salt in water represents the amount of solute necessary to reach a state of equilibrium between saturated solution and undissolved additional solute" Explain
This is saying that when the solubility of a salt in water is being discussed, that means that they are talking about how much of a substance would be needed to be added to the solvent in order to reach the equalibrium. Equalibrium would be achieved when the solution and solute are balanced. Solubility means the amount of solute needed, to reach the equalibrium, or create balance between the solution and the solute. When there is no more salt that is able to be dissolved into the solvent and chemical changes stopped occuring, that is when the equalibrium occurs.
E6Q7 When adding water to the salt initially, you attempted to find the minimum amount of water the salt would dissolve in at 100 C. Why was it necessary that the solution to be used be almost saturated?
becasue we are trying to find a minimum amount needed, and we should try to avoid going over or adding too much water.
E1Q4 Why should liquids never be poured in the vicinity of a balance?
because if any liquid gets on a balance, even in a really small amount, it can ruin the results by affecting the mass. Chemical or othersubstance spills can also damage electric balances due to the moisture.
E6Q6 Why is it better to determine the saturation temp while the temp is dropping, rather than while it is still rising?
because if you determine the saturation temp while the temp is still rising, then the solute can still be dissolving and changing the final solutions, so it could lead to errors if the temp is not taken while it is dropping. It cools at a constant rate.
E4Q6 why is the ice/water bath used to calibrate the thermometer at 0 C referred to as an "equalibrium system"?
because it is a balance between the ice which may be a certain number of degrees below 0 C and the water which is above 0 C, but the ice in the water creates the balance of the two parts at the 0 C temp. It is going between the solid to liquid and liquid to solid states.
E4Q1 Why is a mixture of ice and water, rather than ice alone, used in calibrating a thermometer?
because of the temperature present within that mixture. IF the thermometer was calibrated in solid ice, the temp could be lower than 0 C, which is what is needed to calibrate. When ice and water are put together in a mixture, they reach an equalibrium. Some parts are going from a solid to a liquid, some parts are going from a liquid to a solid.
E4Q7 Why is the temp of the ice/water bath not dependent on the barometric pressure, as was the temp of the boiling water bath.
because over all of the everyday pressures, ther isnt going to be any large changes that would affect temp, like it did with boiling a gas. It is virtually incompressible, so not as affected by pressure changes.
E1Q5 Why do we typically determine the mass of objects in the lab by a difference method?
because sometimes there are small errors in the balances. These errors can occur if the balance has not been properly calibrated or it the balance has been abused.
E7Q1 Why are the freezing point of a liquid and the Melting point of a solid the same temperature for a given substance?
because that degree value is that pint where it is at a state of equilibrium. the temp at which it goes from a solid to a liquid or a liquid to a solid is the same since the equilibrium also has to be the same rate.
E4Q8 Usually, we fire polish glass to make sure it does not have rough edges that might cut us during use of the glass. Why was it desirable in this experiment that the capillary tube should have a rough surface at the end.
because the capillary tube has a nucleated edge which surves as a surface for the bubbles to form and stick to during boiling.
E8Q6 Why should the flask containing a mizture to be distilled never be heated to complete dryness?
because the flask could then break from the heat that is being applied to it. When the distillation is complete, put a stopper over the flask and be done.
E6Q3 Why does stirring affect the rate at which a salt dissolves in water, but not the solubility of the salt in water?
because the speed has nothing to do with the final max quantitiy of solute that can be dissolved in a certain amount of solvent. In other words, stirring just disperses and moves the salt around, which in turn makes it all more exposed. Water is polar and attracts the ions which dissosiate. Ions leave solid and go liquid or vise versa, which creates the equalibrium.
E4Q2 Why does the boiling point of a liquid vary with the barometric pressure?
because the temp at which the vapor escaping from the surface of the liquid has a pressure equal to the pressure existing above the liquid. In most cases, the liquid being tested woul dbe sitting out in a room, and the barometric pressure for that day would be the pressure that is acting upon it. IF the barometric pressure is lower, the temperature needed to go to vaporization will be lower.
E3Q6 Why was it necesary to detemine the temperature during the determination of the density of the sodium chloride solutions? which factor-mass or volume- used in calculating the density would be affected by temperature? Explain.
because the temp would have an impact on volume, which would therefore change the density.
E8Q4 Why is a fractionating colum packed with small glass beads or short pieces of glass tubing? How does this help improve a distillation?
because they then provide a large amount of surface area to the substance being boiled. This causes the substance to be re-distilled multiple times, which improves the distillation.
E8Q3 (methods) Magnetism
ideal for seperating mixtures of two solids, one part with magnetic properties. Examples include iron, nickel, and cobalt.
E5Q4 Suppose when you prepare to determine the melting point of your sample, you discover that the oil in the Thiele tube is cloudy. What does this mean? What should you do about it? Why?
if the oil is cloudy, the the temp is too high or the test tube is dirty. If this happens, then you should replace it so that you can properly use it and have measurements that are accurate. If it is cloudy, it could indicate that there is moisture present which could cause splatter when it comes in contact with hot oil
E5Q3 What is meant by a mixed melting point determination?
is measuring the melting point of a substance and confirming what substance you think it is by mixing it with a known sample of that substance. If the entire mixture melts at the same itme, then its likeluy that the same substance. Its called mixed because you are using two different samples. You need both melting points in order to determine.
E5Q1 What does it mean to say that the melting point of a pure substance is a characteristic property of the substance?
it is a property of that substance that, when under the same lab conditions, will always have the same melting points. Also means that the melting point can be used to determine the identity of the substance. If it is pure, then it will have the exact melting point that its supposed to have
E1Q1 What are the differences between mass and weight? In the laboratroy, do we determine the mass or the weight of objects?
mass is defined as the direct measure of the amount of matter in a sample of some substance, where as weight is defined as the force exerted on an object due to gravity. In the lab, we determine the mass.
E7Q4 physical properties
properties that don't involve chemical changes. some examples include boiling point, melting point
E6Q2 solubility
the number of grams of the substance that will dissolve in 100 g of the solvent, at a particular temp
E8Q1 (methods) chromatography
the seperation of a mixture by passing it in a solution, suspension, or vapor through a medium that the componets move in different rates. inner molecular interactions.
E4Q4 What is meant by the normal boiling point of a substance
the temp at which a liquid's vapor is equal to 1atm.
E3Q3 What error would be introduced to the determination of the density of the irregularaly shaped metal pellets if you had not stirred/shaken the pellet to remove adhering air bubbles? Would the density have been too high or too low?
then the density would have been affected by the volume, increasing due to the presense of the air bubbles. This would lower the density because the volumes getting larger, without the mass increasing.