University:
California State University San Marcos
Course:
CHEM 160 | General Chemistry II
Academic year:

2020
Views:

193

Pages:

3
Author:

Yemima C.

Compound Stoichiometry A. Substance Composition One of the important activities in chemistry is conducting experiments to identify substances. There are two activities in the identification of substances, viz qualitative analysis and quantitative analysis. Qualitative analysis is used for determine the type of components that make up the substance. While quantitative analysis carried out to determine the mass of each component of the substance. By knowing the type and mass of each component making up the substance, we know the composition of the substance. The composition of the substance is expressed in mass percent (% mass). Calculation mass percent for each component can use the following equation. Example 1. A chemist analyzes a number of samples of a substance. he found that the 65-gram sample contains 48 grams of carbon, 9 grams of hydrogen, and 8 grams of oxygen. Express the composition of the substance in mass percent! Answer: Example 2. Analysis of the sample showed that it contained 40% calcium, 12% carbon and 48% oxygen. If it is known that the mass of the sample is 25 grams, determine the mass of each element in the sample! Answer: B. Theoretical Composition of Substances The composition of the substance is theoretically the composition of the substance determined from the chemical formula. For substances in the form of compounds, the composition is theoretically It can be expressed as a percent by mass of the elements in the compound. with: Ar = relative atomic mass (gram/mol) Mr = relative molecular mass (gram/mol) Exercise 1 1. One sample of a substance contains 2.4 grams of carbon, 3.2 grams of oxygen, 5.6 grams of nitrogen and 0.8 grams of hydrogen. Express the composition of the substance in mass percent! 2. The elements nitrogen and oxygen react, and can form more than one kind of compound. Determine the percentage of nitrogen and oxygen elements in the compounds NO, NO2 , N2 O, N2 O3 , and N2 O5 (Ar N = 14 and O = 16)! 3. Determine the levels of C and N in urea (CO(NH2 )2 )(ArC = 12, O = 16, N = 14, and H = 1)? 4. What is the mass of oxygen needed to make 500 kg of water (Ar H = 1 and O = 16)? 5. What mass of calcium is contained in 250 kg of CaCO3 (Ar Ca = 40, C = 12, and O = 16)? 6. What is the mass of sodium chloride (NaCl) which contains 196.58 grams of sodium (Ar Na = 23 and Cl = 35.5)? 7. What is the mass of phosphoric acid (H3 PO4 ) made from 6.2 grams of phosphate (Ar H = 1, P = 31, and O = 16)? 8. An amino acid with a relative molecular mass (Mr ) of 68,000 contains 0.33% iron (Ar Fe = 56). Count the number of iron atoms in each hemoglobin molecule! 9. An amino acid with a relative molecular mass (Mr ) of 146 contains 19.2% nitrogen. (ArN = 14). How many nitrogen atoms are in the acid molecule?