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Mass Percent, atom percent, mass fraction, elemental fraction

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To do any mass percent question one must first understand a little about fractions. If one has a fruit basket, with 3 apples, 2 oranges, 4 bananas, and a grapefruit, we could say that there is %30 apples in the basket, %20 oranges and so on if we were talking about the percentage by fruit.

If we were talking about percentage by mass we would need to know the masses of the individual fruits, as well as the total mass (which we could get from the former).

This is the same thing when we talk about molecules, or chemical compunds. The question was not given, but I will give an example that you can hopefully follow....

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If I have a compound that has a formula C2H6O1, and I were asked to do the atom percent, I would take the number of atoms of each type, and divide by ...

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The expert examines mass percent, atom percent, mass fractions and elemental fractions. Molecules and chemical compounds are given.

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Chemistry review questions

Question 2
An imaginary element X consists of two isotopes having masses of 110.00 amu and 112.00 amu. ("amu" is an atomic mass unit.) A sample of X was found to consist of 79.94% 110X and the rest is 112X. What would be the average atomic mass of X in this sample?
Give your answer to four significant figures. Do not enter any units in the box below.

Question 6
A compound is composed of 52.2% carbon, 34.8% oxygen and 13.0% hydrogen, by mass. What is the empirical formula of the compound?

a. ) CHO

b. ) C2H6O1

c. ) C12H22O11

d. ) C6H11O7

e. ) CH2O3

Question 7
Identify the type of reaction expected, if any, for each of the following pairs of reactants..
Assume there is enough energy input to get the reaction going, if it is favourable at room temperature (i.e. consider thermodynamics, not kinetics). If the reverse reaction is preferred, assume no reaction.

Preview columns:
â?¢ NaHSO4(aq) + CH3NH2(aq)
â?¢ K3PO4(aq) + NiCl2(aq)
â?¢ Pb(NO3)2(aq) + Zn(ClO4)2(aq)
â?¢ P4(s) + O2(g)
â?¢ Mn(s) + NiCl2(aq) â?¢ precipitation
â?¢ acid base
â?¢ no reaction
â?¢ oxidation reduction

Question 13
Select the net ionic equation for the reaction which occurs when aqueous solutions of mercurous acetate and sodium iodide are mixed :

a. Hg2(CH3COO)2 (aq) + 2 NaI (aq) --> 2 HgI (s) + 2 CH3COONa (aq)

b. Hg22+ (aq) + 2 I- (aq) --> Hg2I2 (s)

c. 2 Na+ (aq) + 2 CH3COO- (aq) --> 2 CH3COONa (aq)

d. Na+ (aq) + CH3COO- (aq) --> CH3COONa (aq)

e. Hg+ (aq) + I- (aq) --> HgI (s)

Question 18
Calculate the volume (in mL) of 0.567 M HCl(aq) required to react with 46.6 mL of 0.383 M Na2CO3(aq).
Be careful to enter the correct number of significant figures. Do not enter any volume units!


Question 21
From the following species (molecules and/or ions) choose the one that has the largest (most positive) oxidation number for the underlined atom.

a. C2H6

b. H2SO4

c. NO2

d. HIO3

e. XeF4

Question 29
What is the oxidizing agent in the following spontaneous reaction?
OCl-(aq) + I2(s) + 2 OH-(aq) --->Cl-(aq) + 2 OI-(aq) + H2O(l)

a. ) I2

b. ) OCl-

c. ) OI-

d. ) OH-

e. ) Cl-

Question 30
Which two of the statements are FALSE regarding the balanced equation, corresponding to the following unbalanced reaction?
Co2(SO4)3(aq) + Pb(s) --> CoSO4(aq) + PbSO4(s)

a. ) Cobalt decreases its oxidation number from +3 to +2 in this reaction.

b. ) Co3+(aq) is the oxidizing agent in this reaction.

c. ) Co2+(aq) is a stronger reducing agent than Pb(s).

d. ) One mole of Co2(SO4)3(aq) consumes one mole of Pb(s).

e. ) Co3+(aq) is a weaker oxidizing agent than PbSO4(s).

Question 10
When copper is heated with an excess of sulfur, Cu2S is formed. How many grams of Cu2S could be produced if 250. g of copper is heated with 100. g of sulfur?

a. 626. g

b. 125. g

c. 156.5 g

d. 313. g

e. 532. g

Question 11
In the direct reaction of silicon with Cl2 the yield of SiCl4 is 40 percent. How many grams of SiCl4 are obtained in the reaction of 2.8 g of silicon with excess chlorine?

a. 170 g

b. 17 g

c. 34 g

d. 6.8 g

e. 3.4 g

Question 12
Solid glucose, C6H1206(s) is burned in excess oxygen in a closed container. After the reaction, the total gas volume is 10.0 L at a temperature of 298K. What is the partial pressure, in atmospheres, of the CO2(g) produced by the complete combustion of 5.1 grams of glucose?
Be sure to enter the correct number of significant figures, but do not enter the units (atm)

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