Unit-I Practice Problems
1. When 0.273 g of Mg is completely reacted at high temperatures with N2 to produce 0.378 g of a solid, MgxNy.
a. Calculate the empirical formula of the product.
b. How many L of N2 (g) are used up in this reaction at standard temperature and pressure (STP)? (1 mol of any gas = 22.4 L at STP.) Ans.
2. An unknown compound, X, contains carbon, hydrogen, and nitrogen. Upon analysis it is found to contain 60.0 percent carbon. When 4.00-gm of the compound is burned, all of the hydrogen is converted to 0.033-moles of water.
a. What is the weight of each element in a 4.00-g sample of X?
b. What is the simplest formula of X?
c. Given that the molar mass of the compound is 362 g/mol, what is its molecular formula?. Ans.
3. Manganese trifluoride, MnF3, is used to make fluoro-chemicals. It can be prepared by the following reaction.
2 MnI2 (s) + 13 F2 (g) ---> 2 MnF3(s) + 4 IF5(l)
What is the minimum number of grams of F2 that must be used to react with 12.0 g of MnI2 if the overall yield of MnF3 is no more than 75% of theoretical? Ans.
4. The molecular formula of a hydrocarbon is to be determined by analyzing its combustion products.
a. The hydrocarbon burns completely, producing 7.2 grams of water and 7.2 liters of CO2 at standard conditions. What is the empirical formula of the hydrocarbon?
b. If the molecular weight is 58.14, what is the molecular formula of the hydrocarbon?
c. Calculate the mass in grams of O2 required for the complete combustion of the sample of the hydrocarbon described in (a). Ans.
5. Qualitative analysis revealed that an unknown contained the anions, NO3-, Cl -, and ClO3-. A student weighed a sample into a beaker and recorded the following data:
Weight of beaker and sample 68.962 grams
Weight of beaker 67.625 grams
The sample was dissolved and diluted to 200.0 milliliters. A 50.00 milliliters portion of the solution was treated with excess silver nitrate solution, and the precipitate was collected in a filter crucible. Weighing gave the following data:
Weight of crucible and AgCl 10.211 grams
Weight of crucible 10.102 grams
Another 50.00-milliliter portion was treated with a reducing agent before the addition of excess silver nitrate solution. The precipitate which formed, was collected and weighed in another filter crucible.
Weight of crucible and AgCl 10.247 grams
Weight of crucible 10.110 grams
a. Determine the percentage of Cl - in the unknown.
b. Determine the percentage of ClO3- in the unknown. Ans.
6. The compound UF6 reacts with water to form a solid compound and a gas. The gas is 95 per cent by mass fluorine and 5 per cent by mass hydrogen, and its molecular formula is assumed to be the same as it empirical formula.
a. What number of moles of gas is formed?
b. What number of gram-atomic weights of fluorine is removed from one mole of UF6?
c. What is the empirical formula of the solid compound? Ans.
7. A compound of Ca, C, N, and S was subjected to quantitative analysis and formula mass determination, and the following data were recorded. A 0.250-g sample was mixed with Na2CO3 to convert all of the Ca to 0.160 g of CaCO3. A 0.115-g sample was carried through a series of reactions until all of its S was changed to 0.334 g of BaSO4. A 0.712-g sample was processed to liberate all of its N as NH3, and 0.155 g NH3 was obtained. The formula mass was found to be 156 g/mol. Determine the empirical formula of this compound. Ans.
8. In dilute nitric acid, HNO3, copper metal dissolves according to the following equation:
3 Cu(s) + 8 HNO3(aq) ---> 3 Cu(NO3)2(aq) + 2 NO(g) + 4 H3O .
How many moles of HNO3 are needed to dissolve 11.45 g of Cu according to this equation? Ans.
 Back to AP Chemistry Page |
Yahoo Quick Search
Send E-Mail to: pthaler@simi.k12.ca.usCopyright © 2009 . All Rights Reserved