CHEM 121 LAB TECHNIQUES EXAM QUESTIONS AND ANSWERS WITH COMPLETE SOLUTIONS VERIFIED GRADED A+ 2024/2025
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Course
CHEM 121
Institution
CHEM 121
CHEM 121 LAB TECHNIQUES EXAM QUESTIONS AND ANSWERS WITH COMPLETE SOLUTIONS VERIFIED GRADED A+ 2024/2025
After what seconds is the number on balance considered stable?
reading is considered stable if it persists for 2 seconds
Top-Loading Balance
- pre-weighing samples, not precise
- +/- 0.1g ...
CHEM 121 LAB TECHNIQUES EXAM QUESTIONS AND
ANSWERS WITH COMPLETE SOLUTIONS VERIFIED
GRADED A+ 2024/2025
After what seconds is the number on balance considered stable?
reading is considered stable if it persists for 2 seconds
Top-Loading Balance
- pre-weighing samples, not precise
- +/- 0.1g or +/-0.01g
- uncertainty of +/-0.05g
- range of +/- 0.1g from the desired mass is acceptable
Analytical Balance
- precise
- +/- 0.0001 g (4 decimal places)
- uncertainty of +/- 0.00005g
- air currents may lift the pan so always keep the doors closed
What happens when you weigh objects while it is still HOT?
the object will create convection current in the air around the balance pan.
this fluctuating forces reduces the air pressure on the balance pan and make it difficult
to obtain a stable reading.
Why should you only weigh DRY objects?
moisture can corrode the balance pan as it evapourates which causes an unstable
reading
*ultra dry objects may absorb moisture from atmosphere (or breathing on the object
condenses moisture on it)
Why should you use the SAME balance?
, each devices may have been calibrated slightly differently.
Weigh Boats
weigh boats aer containers used to prevent reagents from contacting the balance pan.
they are made with polypropylene (plastic) that does not absorb water
What happens when you handle glass objects with bare fingers?
it deposits oil on them and cause increase in mass. hold object through a strip of paper
or wear latex gloves
Weighing by Difference
the full amount of weighed object is almost never completely transferred to a beaker.
(total mass)-(empty mass)=(mass transferred to the beaker)
What is filtration?
separation of solid from a liquid by pouring the mixture through a membrane with small
holes (pores). the relatively large solid particles cannot pass through the membrane
whereas the liquid, comprised of tiny molecules is able to pass through)
Two common types of Membranes
1. filter paper
2. sintered glass crucibles
Two types of filtrations (also describe them)
1. quantitative filtration = when trying to collect every sinlge particle of solid material
produced in a chemical reaction
2. Preparative filtrations = performed when the requirement above is not needed
Gravity filtration
Preparative, uses glass funnel and filter paper
Preparative Vacuum filtration
uses buchner funnel and filter paper
Quantitative Vacuum filtration
uses sintered glass crucible
Quantitative Analysis
determination of the amount or percentage of one or more constituents of a sample
Quantitative Filtration (requirements)
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