Zinc is a bluish-white, moderately hard metal. It is brittle at room temperature and at temperatures above 150°C, being workable only in the range between 100°C and 150°C. It is an active metal and will displace hydrogen even from dilute acids.
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It provides atomic mass, mass excess, nuclear binding energy, nucleon separation energies, Q-values, and nucleon residual interaction parameters for atomic nuclei of the isotope Zn-62 (Zinc, atomic number Z = 30, mass number A = 62). For other uses, see Zinc (disambiguation). Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a silvery-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic table. Another green zinc mineral is the zinc arsenate adamite, Zn 2 AsO 4 (OH). There is a dimorph of adamite called paradamite. The zinc arsenate legrandite has the same empirical formula as adamite exept for an added water of hydration and has a dramatic orange color. The zinc arsenate minerals reinerite, Zn 3 (AsO 3) 2, and kottigite lack the. Zinc is a chemical element with symbol Zn and atomic number 30. Classified as a transition metal, Zinc is a solid at room temperature.
The principal use of zinc is for the galvanizing of iron sheets or wires. In moist air zinc oxidizes and becomes coated with a tough film of zinc carbonate which protects it from further corrosion. Galvanizing is done by dipping the iron into molten zinc. Zinc can also be electroplated onto iron pieces, giving a smoother galvanized surface. Sherardized iron is formed by coating the iron with and iron-zinc alloy and then baking at 800°C. The zinc coating on iron offers protection even if the zinc coating is broken by means of cathodic protection. The zinc has a more negative reduction potential (-.76V) than iron (-.04V) and therefore acts as an anode and oxidizes in preference to the iron.
Zinc is used in making alloys such as brass (the alloy with copper). Zinc is used as the outside electrode in dry cell batteries It was used in early voltaic cells such as the Daniell cell.
Zinc sulfate is used as a disinfectant and as a white pigment in paints.
Zinc oxide (ZnO) is also used as pigment called zinc white, and is used in antiseptics (zinc oxide ointment for burns). Zinc oxide is used in making rubber to improve the mechanical characteristics of the product. It is used as a constituent of the photoconductive surfaces in photocopiers. Zinc is found with manganese and iron in the oxide mineral Franklinite. Accompanying Franklinite may be the oxide mineral of zinc with calcium Hardystonite. One of the major zinc ores is the red oxide with manganese and/or iron named zincite. An oxide of zinc with manganese is Hetaerolite.
The most common zinc ore is sphalerite, ZnS. Zinc metal is produced by first roasting the ore in air to form its oxide
2ZnS(s) + 3O2(g) -> 2ZnO(s) + CO(g).
The zinc oxide is then reduced by heating it with coke (carbon) which produces the metal vapor. The vapor is condensed to the liquid form and cooled to form the solid metal
ZnO(s) + C(s) -> Zn(g) + CO(g).
The fact that the temperature required for reduction is above the boiling point for zinc (907°C) delayed the discovery of zinc. Similar reductions were used to prepare copper, lead and iron even in ancient Rome. Zinc was probably also produced, but was lost to vaporization. Brass was formed several centuries before the discovery of zinc as a pure metal (brass is a mixture of copper and zinc).
In addition to the sphalerite, zinc joins with iron to form the sulfide wurtzite, (Zn,Fe)S. Another naturally occuring ore of zinc is a carbonate mineral called Smithsonite. A striking green color is exhibited by the mineral bayldonite which combines copper, zinc and lead with an arsenate group. Another green zinc mineral is the zinc arsenate adamite, Zn2AsO4(OH). There is a dimorph of adamite called paradamite. The zinc arsenate legrandite has the same empirical formula as adamite exept for an added water of hydration and has a dramatic orange color. The zinc arsenate minerals reinerite, Zn3(AsO3)2, and kottigite lack the dramatic color.
Zinc forms the silicate willemite, Zn2SiO4 which can also produce green crystals. Zinc also forms the silicate hemimorphite. Zinc with manganese forms the silicate Hodgkinsonite. Zinc with magnesium, manganese and arsenic forms the silicate Mcgovernite. Zinc with aluminum forms the oxide mineral gahnite. Zinc is found with manganese and iron in the oxide mineral chalcophanite. An oxide with lead and vanadium is descloizite. An oxide of zinc with copper, vanadium and lead is mottramite
Zinc phosphate, Zn2PO4(OH), forms the mineral tarbuttite. Copper appears with zinc in the phosphate mineral veszelyite. Zinc appears with manganese and iron in the phosphate mineral phosphophyllite. Calcium appears with zinc in the phosphate mineral scholzite.
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Experiment
10
Overview
In this experiment, you used a gas buret to measure the volume of hydrogen gas that was evolved when a small piece of magnesium or zinc was reacted with acid. From the volume of hydrogen that was collected, you should be able to calculate the number of moles of hydrogen that was evolved, and from this (and the stoichiometry of the reaction), the number of moles of Mg or Zn that must have been present in the sample you took. Since you also weighed the sample of Mg or Zn, you can calculate the atomic weight of the metal you used (g/mole).
Data
Suppose the following data were recorded for Trial 1 of the experiment (we won't bother with data for the other Trials, but your own data table should be for all three Trials).
Metal Used | Mg |
Mass beaker + sample | 10.2011 g |
Mass empty beaker | 10.1243 g |
Initial reading of gas tube | 2.25 mL |
Final reading of gas tube | 79.31 mL |
Temperature of hydrogen gas | 24oC |
Difference in water levels | 35.2 mm H2O |
Temperature of water in trough | 23oC |
Density of water at temp. of trough | 0.99751 g/mL (from Appendix) |
Aqueous vapor pressure at water temp. | 21.1 mm Hg (from Appendix) |
Barometer reading | 759.9 mm Hg (from Instructor) |
Barometer temperature | 24oC (from Instructor) |
Barometer scale correction | 3.00 mm Hg (from Appendix) |
Calculations
Page 106, Part II
1. Mass of Sample
The mass of metal sample used is just the difference between the masses of the empty beaker and the beaker with the sample in it.
Mass of Sample = 10.2011 g - 10.1243 g = 0.0768 g |
2.Volume of Gas Collected
The volume of hydrogen gas just represents the difference in volume levels in the gas tube before and after the reaction. For my data
Volume of gas collected = 79.31 mL - 2.25 mL = 77.06 mL |
3. Corrected Barometric Pressure
Remember that the apparent height of the mercury column in a barometer is also affected by the temperature. The Appendix to the lab manual lists corrections for barometeric readings for different temperatures. For my data above, the barometer was at 24oC and had a mercury height of 759.9 mm Hg. The correction listed in the Appendix for these conditions is 3.0 mm Hg. So the correced barometric pressure is
Corrected barometric pressure = 759.9 mm Hg - 3.0 mm Hg = 756.9 mm Hg |
4. Mercury Equivalent of Difference in Water Levels
You'll remember that in this experiment you had to measure in millimeters, with a ruler or meterstick, the difference in height between the level of the water in the gas tube and that in the trough or beaker. Since there was liquid water inside the gas tube, the column of liquid water contributes a portion of the total pressure inside the gas tube. The total pressure inside the gas tube (which is equal to the atmospheric pressure) is composed of: the pressure of the hydrogen gas (what we want!), the pressure of water vapor which is mixed with the hydrogen gas, and the pressure due to the column of liquid water. For my data above, I found that the column of liquid water in the gas tube had a height of 35.2 mm above the surface of the water in the trough. Since we measure the pressure of gases in units of mm Hg (torr), we can convert the pressure due to the 35.2 mm of water into the equivalent level of Hg which would have the same pressure, by using the densities of mercury (13.596 g/mL) and water (0.99751 g/mL at the temperature of my experiment.
5. Pressure of Hydrogen gas
The total pressure inside the gas tube (which is equal to the atmospheric pressure) is composed of: the pressure of the hydrogen gas (what we want!), the pressure of water vapor which is mixed with the hydrogen gas, and the pressure due to the column of liquid water.
That is Ptotal = Patmosphere = Phydrogen + Pwater vapor + Pliquid water
So if we subtract the pressure of water vapor (21.1 mm Hg at the temperature of my experiment) and the pressure due to the column of liquid water (2.58 mm Hg as calculated in Part 4 above) from the total corrected barometric pressure (756.9 mm Hg from Part 3 above), this will give us the pressure of the hydrogen gas itself.
Phydrogen = 756.9 mm Hg - 21.1 mm Hg - 2.58 mm Hg = 733.2 mm Hg |
6. Absolute temperature of Hydrogen gas
The Celsius temperature scale is a human invention, and we can't expect hydrogen gas molecules to know about it. The temperature of my hydrogen sample was 24oC, which has to be converted to Absolute (Kelvin) degrees:
7. Moles of Hydrogen Produced
All of the above calculations have been leading us to being able to figure the amount of hydrogen generated using the ideal gas law (PV = nRT). Fill in the following data and solve for n -- the number of moles of hydrogen:
- P = 733.2 mm Hg (733.2 torr)
- V = 77.06 mL (0.07706 L)
- R = 62.358 L torr/mol K
- T = 297 K
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8. Moles of Metal
For magnesium, the balanced chemical equation is Mg + 2HCl ® Mg Cl2 + H2
Since the coefficients of magnesium metal and hydrogen gas are the same in the balanced chemical equation, if 0.00305 moles of hydrogen gas was produced, then the sample of magnesium reacted also must have consisted of 0.00305 moles.
Zinc 64 Atomic Mass
9. Experimental Gram-Atomic Weight (Molar Mass) of Metal
The gram atomic weight of a metal represents the mass of 1 mole of the metal expressed in grams. More recently, this has been termed the 'molar mass' of the metal. In any case, we want the number of 'grams per mole' for the metal. In Part 1 of the calculations, we determined that the sample of magnesium weighed 0.0768 g. In Part 8 of the calculations, we determined that the sample of magnesium consisted of 0.00305 moles. Putting these together gives the molar mass
Parts 10, 11, 12
By now I'm sure you're an expert at calculating means, deviations, and the average deviation for a set of measurements. If you've forgotten, see the help on Experiments 1, 2, 4, or 9.
Parts 13 and 14
Zn Element Atomic Mass
The actual gram atomic mass (molar mass) of magnesium is 24.31 g. In Part 14, you calculate how far off your answer is on a percentage basis. For my data, the % error is
Questions
Density Of Zinc
We can't solve the problems for you, but here are some hints as to how to go about the task.
Zinc Atomic Mass In Kg
- This question is essentially the same as the calculations above, only going backwards. Plus, you don't have to worry about all the corrections to the pressure, since the pressure given (720 torr) can be assumed to be the pressure due to the hydrogen alone. Use the ideal gas equation (PV = nRT) with the given data to figure out n (which will be the number of moles of hydrogen gas that is present in the sample). Then use the balanced chemical equation to figure out how many moles of Al metal would have to react to produce the calculated amount of hydrogen (make sure the equation is balanced: this is not a 1:1 reaction). Finally, use the atomic mass of Al to figure out the mass of Al for the calculated number of moles. The answer is 0.045 g, but you'll have to show a complete solution to receive credit on your report.
- First figure out how many moles of K is present in 0.0379 g K using the molar mass of K. Then, use the balanced chemical equation to figure out how many moles of hydrogen would result from this amount of K reacting with HCl. Finally, use the ideal gas equation (PV = nRT) filling in this number of moles, along with the conditions represented by 'STP'.
- This one is a killer!! You are really going to have to use your skills in algebra to solve it. Approach the problem this way: let x represent the mass of Mg in the sample and let y represent the mass of Zn. We know that (x + y) = 0.1000 g. From the volume of hydrogen present at STP, we can calculate the number of moles of hydrogen that the mixture generated using the ideal gas equation. Since both Mg and Zn produce hydrogen on a 1:1 stoichiometric basis (from the balanced chemical equations), the number of moles of hydrogen produced is equal to the total number of moles of Mg and Zn combined. How are the number of moles of Mg and Zn related to their molar masses? In terms of the x and y variables we have defined, the number of moles of Mg is (x/24.31) and the number of moles of Zn is (y/65.38). The rest of the calculation is left to you......!!!