Laboratory Techniques Flashcards
what are beakers?
glassware - the wide, thick cylinders
has a spout at the top rim that makes it easier to pour liquids
useful for heating, mixing, or stirring solutions
they are not very accurate and therefore, are NOT a good choice for measuring volumes
what are graduated cylinders?
one of the go-to types of equipment for measuring the volume of a liquid. BUT there are better tools available for situations where you need a high degree of accuracy (pipettes, burettes, volumetric flasks)
are narrower and taller than beakers, and have graduated markings that indicate volume levels
what equipment CAN be used to measure volume of liquids? list them in order of precision
beakers/erlenmeyer flasks
graduated cylinders
burettes/volumetric flasks
pipettes
what are test tubes?
glass tubes with one open end and a round, closed end
holds small volumes of samples and can also be used for mixing
primarily used to compare different samples based on qualitative measures such as color
can be heated and cooled or used to store a solution over a period of time.
should NOT be used to measure volumes
what are droppers?
small glass tubes with a rubber
bulb on one end
used to deliver a liquid (usually an indicator), in a drop-wise fashion
also known as Pasteur pipettes
what are erlenmeyer flasks?
a type of conical flask with a narrow neck and a wider base
like the beaker, Erlenmeyer flasks can be used for mixing, heating, and stirring solutions. but typically preferred over beakers for mixing and swirling solutions as their shape minimizes the risk of spilling and splashing
are NOT very accurate in measuring volumes
also used during titrations
what are volumetric flasks?
round flasks with flat bottoms and long, narrow necks (even thinner than erlenmeyer)
typically used to obtain a very specific
volume of a liquid, which is indicated by a small marking on the neck of the flask
a cap is commonly used to seal the top of the flask to prevent evaporation and facilitate vigorous shaking
one of the most common uses of a volumetric flask is for preparing or diluting a chemical solution
what is a boiling flask?
have round bottoms and a long
neck
primarily used to swirl and heat
liquids, but due to the nature of the round bottom, a clamp is often required to keep it in position.
they cannot and should NOT be used
to measure the volumes of liquids
what are bunsen burners?
metallic apparatuses that produce an open flame for heating purposes
they are connected to a natural gas source by rubber tubing and have both a valve that controls gas flow and a collar that can be rotated to adjust airflow
the amount of gas that is provided to the Bunsen burner is related to the size of the flame.
a striker is used to light the Bunsen burner and ignite the flame.
what’s a pH meter?
used to accurately measure the pH of a solution
it should always be calibrated prior to
use to ensure accurate measurements
can be used in acid-base titrations or any other experiment where it is critical to record the change in pH
what are digital balances?
essential for accurately measuring the mass of a solid sample
the “tare” button on a digital balance
resets the reading to “0”, ensuring that the current weight of any object on the scale is not included in the final measurement
what are burettes?
highly accurate and are used to
dispense very precise volumes of a liquid
long, graduated glass tubes with a
very narrow opening at the bottom
a stopcock is positioned right above this opening and is used to control the amount of liquid that is released by the burette
commonly used in titrations to deliver a titrant to a solution
what is a titrant?
a solution of known concentration that is added to another solution of unknown concentration in a process called titration. this allows for the determination of the unknown solution’s concentration
what are pipettes?
can be graduated, volumetric, or
electronic and are the most precise glassware for measuring the volume of a liquid
they come in a wide range of sizes but are ultimately used to transfer a specific volume of liquid to a container
what is centrifugation? explain the process. what’s decantation?
a process by which a centrifuge is used to separate different parts of a mixture
they are most commonly used to separate a solid pellet from the liquid supernatant, which lies on top
the centrifuge spins the mixture at very high speeds and the resulting centrifugal force acts upon the denser components of the mixture (the solids) such that they move towards the bottom of the vessel and form the pellet
after the centrifugation process has finished, we can simply separate the liquid supernatant from the solid pellet by using either a pipette or by decanting the liquid. in some cases, filtration can also be used
simple decantation involves pouring the liquid out of the original vessel, leaving the solid behind
what is calorimetry?
calorimetry is a method used across general chemistry labs to measure the amount of heat energy that is released or absorbed during a chemical reaction
this change in heat can help determine whether a reaction is exothermic or endothermic. an exothermic reaction
releases heat while an endothermic reaction absorbs heat
a basic calorimetry setup has a Styrofoam cup sealed by a lid, and a thermometer to measure the temperature
what is titration? what are the following terms: equivalence point, end point, and buffer zones?
a method that delivers drop-by-drop volumes of a titrant to another solution through a burette to measure the volume of the titrant needed to react completely with the other reagent
the point at which the added titrant has completely reacted with the reagent is known as the equivalence point
the endpoint is the point at which an observable property (e.g. color change) occurs and this is associated with either being at or near the equivalence point
any deviation of the endpoint from the
equivalence point is known as titration error.
the pH of a solution can also be
observed throughout the titration to identify periods of stabilization known as buffer zones.
what’s something to note about eyewear and goggles?
avoid wearing contact lenses in the lab, even with safety goggles, as dangerous fumes can concentrate under the goggles and cause complications
when should a fire extinguisher be used in the lab?
extinguishers should only be
used for solvent or electrical fires and
not those resulting from paper or wood
how should solutions be disposed of in the lab?
disposal of solutions: acids, alkalis, or
other water miscible solutions can be
poured down the drain with water.
organic and other water immiscible
chemicals should NOT be flushed down
the drain and instead disposed of in the
appropriate waste bin
in a lab, what should be noted about mixing water and acid?
don’t add water directly to
concentrated acid: adding water to
acid can cause the acid to splash out
and cause harm to those nearby (you can add concentrated acid to water tho)
differentiate between accuracy and precision
accuracy refers to how close a
measured experimental value, such as the melting point of a compound, is to the actual literature value for that compound
precision refers to the consistency of the measured experimental values such that all values are relatively close to each other
what are the (4) rules to determine what constitutes a significant figure?
All non-zero numbers are significant (ex.
“3”0.”2”0)
Zeroes that are between two non-zero
numbers are also significant (ex. 3”0”.20)
Trailing zeros after the decimal point are
significant (ex. 30.2”0”)
All other zeroes are NOT significant e.g.
5”00”, “0.00”3
what are the rules for multiplying/dividing and adding/subtracting significant figures?
multiplying or dividing: simply express the final answer to the lowest degree of significant numbers based on the numbers that are being multiplied or divided. e.g. let’s say we wanted to divide 550 by 10.0 – since the lowest
number of significant digits is 2 (from 550), we would express our answer with two significant digits to get an answer of 55.
addition and subtraction: write out the answer to these calculations with significance based on the digit of least accuracy. let’s say we wanted to add 34 + 240 + 456.4. the first number is accurate to the ones place, the second number is accurate to the tens place, and the last number is accurate to the tenths place. so, we would need to round our final answer to the tens place. the final answer would be 730 with two significant digits
differentiate between systematic and random error
systematic errors: consistently skewed in one direction - either above or below the true value, and to the same extent each time. they are often the result of faulty equipment or an error in the procedure. e.g. forgetting to tare the electronic balance before taking the measurements. the mass of the weighing
paper would be consistently added to the mass of the sample across all measurements
random errors are errors that are skewed in either direction - both above and below the true value, and to a different extent each time. these are often the result of human error such as discrepancies in reading the meniscus on a piece of glassware. if our eyes are not level to the plane of the meniscus, we
may read the volume of the liquid as being higher or lower than what it is.
how do you find the percent error of an experimentally derived answer?
% error = (|experimental − theoretical|) /
theoretical
