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2. Prior to 1961, the molar atomic mass of the oxygen-16 isotope was taken as exactly 16 g/mol by physicists. Chemists actually used a different value. In 1961 the molar atomic mass of carbon 12 was defined to be exactly 12 g/mol (a value agreed upon by chemists and physicists) and the molar atomic masses of all the elements were re-evaluated according to this new standard. This made the re-evaluated molar mass of oxygen-16 become 15.994915 g/mol. Avogadro's number is defined as the number of atoms in the molar atomic mass of an element. The present accepted value for Avogadro's number is 6.0221367 x 1025 /mol. What was the value of Avogadro's number used by physicists before 1961? 72
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Related questions
I am doing the Atomic Mass of Metallo
You may have noticed on the periodic table that the atomic mass of an element is usually not a whole number. That happens because of isotopes. An atom that is missing a neutron or has an extra neutron is called an isotope. They are still the same element; however, they are just a little different from every other atom of the same element.
Most of the carbon atoms in the universe are Carbon-12, with 6 neutrons. A small percentage of carbon atoms are Carbon-13, with 7 neutrons, and an even smaller percentage are Carbon-14 and have 8 neutrons. Carbon-13 and Carbon-14 are isotopes of carbon.
Atomic mass is calculated by determining how many atoms of each isotope type are present in the universe. Since the isotopes of carbon make up only a small percentage of the carbon in the universe, the average of the masses of all carbon atoms is slightly higher than 12, so the atomic mass for carbon is actually 12.011.
Objective
Analyze the isotopes of âMetalloâ and to calculate its atomic mass
Materials
sample of metallo (bag of mixed nuts, bolts, and washers)
balance
Procedures
Obtain a sample of the new element âMetallo.â
Separate the three isotopes of Metallo (nuts, washers, bolts) and measure the mass of each isotope.
Count the numbers of each isotope.
Record all data in the table.
Data Table
Nuts | Bolts | Washers | Totals | |
Total Mass (g) | ||||
Number | ||||
Average Mass (g) | ||||
Percent Abundance | ||||
Relative Abundance | ||||
Relative Mass |
Analysis
Calculate the average mass of each isotope by dividing the total mass by the number of particles of that isotope. Record your answers on the Data Table.
Calculate the percent abundance of each isotope by dividing its number of particles by the total number of particles and multiplying by 100. Record your answers on the Data Table.
Calculate the relative abundance of each isotope by dividing the percent abundance from Step 2 by 100. Record your answers on the Data Table.
Calculate the relative mass of each isotope by multiplying its relative abundance from Step 3 by its average mass. Record your answers on the Data Table.
Calculate the average mass of all Metallo particles by adding the relative masses. This average mass is the atomic mass of Metallo.
Explain the difference between percent abundance and relative abundance. What is the result when you total the individual percent abundances? The individual relative abundances?
The percent abundance of each isotope tells you how many of each kind of isotope exist in every 100 particles. What does relative abundance tell you?
Compare the total values for Rows 3 and 6 in the Data Table. Explain why the totals differ and why the value in Row 6 best represents the atomic mass.
Please give me Post lab questions 2 and 3 at the bottom.
Experiment 7 - Molar Mass of a Volatile Liquid
Introduction. Given the mass of vapor at conditions of known pressure, volume, and temperature, one can determine the molar mass of the vapor. The Ideal Gas Law mathematically relates the quantities of pressure (P in atm), volume (V in liters), and temperature (T in Kelvin) and the quantity of gas (n in moles). Using the expression PV=nRT where R equals the Ideal Gas Constant 0.0821 L*atm/K*mol, one can determine the quantity of gas under given conditions of pressure, volume and temperature. The number of moles of gas is found by
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n =
Because the molar mass (MM) is defined as the mass of one mole of substance in units of g/mol, the molar mass of the vapor can be determined by
MM= mass of vapor in grams/moles of vapor
In this experiment, an unknown volatile liquid is heated in a boiling water bath and is vaporized. The vapor forces air from the flask until the pressure within the flask equals the barometric pressure of the lab. As with all gases, the vapor occupies the entire volume of its container. The temperature of the vapor equals the temperature of the boiling water bath. After 8-10 minutes, the vapor is cooled. The mass of the condensed liquid is determined and the molar mass is calculated as described above.
Notes:
Check out unknown from stockroom
Record unknown number
Before weighing your condensed vapor, thoroughly dry the outside of the flask and make sure that no water is trapped under the foil cover
Do not rinse your flask with water between trials
Dispose of waste in appropriate container
Procedure. You will record your data and calculations in your lab notebook.
Obtain an unknown liquid and a 3 inch aluminum foil square. You will need two 600 or 800 mL beakers, and 3 or 4 boiling chips.
Add 3 or 4 boiling chips to the water in an 800 mL beaker and heat the water to the boiling point. Continue heating for 8 to 10 minutes after the water comes to a rolling boil. The temperature of the boiling water is 100.0oC. This will be the temperature of the vapor. Convert the temperature to Kelvin and record in your lab notebook.
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11.
Chemical Molar Mass
1-chlorobutane 92.6 g/mol
t-butyl chloride 92.5 g/mol
trichloroethylene 131 g/mol
2-propanol 60.1 g/mol
Ethyl acetate 88.1 g/mol
n-hexane 86.2 g/mol
cyclohexane 84.2 g/mol
2-butanone 72.1 g/mol
Post Lab Questions: Answer the following questions using complete sentences. Include them after your conclusion.
1. If a lab group did not keep the flask submerged in boiling water for a full ten minutes, and some unknown never vaporized, how would that affect the calculated molar mass? In other words, would it make the calculated value too high or too low? Explain why.
2. While performing this experiment, ten lab groups have ten different unknown volatile liquids with a range of molar masses. If all lab groups have exactly the same size of flask (255 mL), and all groups took their unknown vapor to the same temperature (100.0oC) and pressure (760.0 mmHg), how would the number of molecules within the different flasks compare? Would it vary with molar mass? How? Explain.
3. A student has determined that an unknown liquid is either 1-chlorobutane or t-butyl chloride. What other physical property(s) could be used to determine the identity of the unknown?