a) Atoms have very small mass and so usually the mass of atoms are given relative to a standard called atomic mass unit.
What is atomic mass unit (amu)?
b) In a reaction A + B2 → AB2, identify the limiting reagent in the reaction mixture containing 5mol A and 2.5mol B.
c) Calculate the mass of NaOH required to make 500 ml of 0.5M aqueous solution. (Molar mass of NaOH = 40)
Answers
Answer:
(b) The limiting reactant is A
Explanation:
(b) the molar ratio between product and reactant is 1:1 so
if 2.5 mol of B2 is used... 2.5 mol of AB2 should have been produced..so..we can see A is excess....
Answer:
Because matter is defined as anything that has mass and takes up space, it should not be surprising to learn that atoms and molecules have mass.
Individual atoms and molecules, however, are very small, and the masses of individual atoms and molecules are also very small. For macroscopic objects, we use units such as grams and kilograms to state their masses, but these units are much too big to comfortably describe the masses of individual atoms and molecules. Another scale is needed.
The atomic mass unit (u; some texts use amu, but this older style is no longer accepted) is defined as one-twelfth of the mass of a carbon-12 atom, an isotope of carbon that has six protons and six neutrons in its nucleus. By this scale, the mass of a proton is 1.00728 u, the mass of a neutron is 1.00866 u, and the mass of an electron is 0.000549 u. There will not be much error if you estimate the mass of an atom by simply counting the total number of protons and neutrons in the nucleus (i.e., identify its mass number) and ignore the electrons. Thus, the mass of carbon-12 is about 12 u, the mass of oxygen-16 is about 16 u, and the mass of uranium-238 is about 238 u. More exact masses are found in scientific references—for example, the exact mass of uranium-238 is 238.050788 u, so you can see that we are not far off by using the whole-number value as the mass of the atom.
Sum = 10.8 u = the atomic mass of boron
Thus, we use 10.8 u for the atomic mass of boron.
For example, if we were to determine the molecular mass of dinitrogen trioxide, N2O3, we would need to add the atomic mass of nitrogen two times with the atomic mass of oxygen three times:
2 N masses = 2 × 14.007 u = 28.014 u
3 O masses = 3 × 15.999 u = 47.997 u
Total = 76.011 u = the molecular mass of N2O3
We would not be far off if we limited our numbers to one or even two decimal places.
Example 5
What is the molecular mass of each substance?
NBr3
C2H6
Solution
Add one atomic mass of nitrogen and three atomic masses of bromine:
1 N mass = 14.007 u
3 Br masses = 3 × 79.904 u = 239.712 u
Total = 253.719 u = the molecular mass of NBr3
Add two atomic masses of carbon and six atomic masses of hydrogen:
2 C masses = 2 × 12.011 u = 24.022 u
6 H masses = 6 × 1.008 u = 6.048 u
Total = 30.070 u = the molecular mass of C2H6
The compound C2H6 also has a common name—ethane.
Test Yourself
What is the molecular mass of each substance?
SO2
PF3
Answers
64.063 u
87.968 u
Chemistry Is Everywhere: Sulfur Hexafluoride
On March 20, 1995, the Japanese terrorist group Aum Shinrikyo (Sanskrit for “Supreme Truth”) released some sarin gas in the Tokyo subway system; twelve people were killed, and thousands were injured (part (a) in the accompanying figure). Sarin (molecular formula C4H10FPO2) is a nerve toxin that was first synthesized in 1938. It is regarded as one of the most deadly toxins known, estimated to be about 500 times more potent than cyanide. Scientists and engineers who study the spread of chemical weapons such as sarin (yes, there are such scientists) would like to have a less dangerous chemical, indeed one that is nontoxic, so they are not at risk themselves.
Sulfur hexafluoride is used as a model compound for sarin. SF6 (a molecular model of which is shown in part (b) in the accompanying figure) has a similar molecular mass (about 146 u) as sarin (about 140 u), so it has similar physical properties in the vapour phase. Sulfur hexafluoride is also very easy to accurately detect, even at low levels, and it is not a normal part of the atmosphere, so there is little potential for contamination from natural sources. Consequently, SF6 is also used as an aerial tracer for ventilation systems in buildings. It is nontoxic and very chemically inert, so workers do not have to take special precautions other than watching for asphyxiation.
Sulfur hexafluoride also has another interesting use: a spark suppressant in high-voltage electrical equipment. High-pressure SF6 gas is used in place of older oils that may have contaminants that are environmentally unfriendly (part (c) in the accompanying figure).
Key Takeaways
The atomic mass unit (u) is a unit that describes the masses of individual atoms and molecules.
The atomic mass is the weighted average of the masses of all isotopes of an element.
The molecular mass is the sum of the masses of the atoms in a molecule.