34 CHEMISTRY FOR AGRICULTURAL STUDENTS 



Conclusions. 



230. State the three laws of chemical combination. Illustrate them by 



examples. 



231. What is meant by an "atom," and how does the assumption of the 



existence of atoms help us to understand each of the three laws of 

 chemical combination ? Illustrate your answer by examples. 



232. What is meant by a " molecule " ? Calculate the molecular weights of 



water, sulphuric acid, and alum, assuming that the formulae given 

 for these compounds represents the number of atoms in each 

 molecule. 



233. The laws of chemical combination have been found to hold good for 



compounds as well as elements. How does the assumption of 

 the existence of molecules help us to understand this ? Illustrate 

 your answer by examples. 



234. What is the constant relation between the relative densities of gaseous 



compounds and their molecular weights ? State the hypothesis 

 which renders this relation intelligible. Upon what grounds do 

 we assign the molecular formulae Hg and Ng to hydrogen and 

 nitrogen ? 



235. The molecular formula of a certain gas is C2Hg. Wliat should be 



its relative density (H= i) ? 



236. The relative density of nitric oxide gas is 15. Is its molecular 



formula NO or NgOg ? 



237. Ozone is described as an allotropic modification of oxygen. Its 



relative density is 24. What is its molecular relation to oxygen 

 gas? 



238. What volume of hydrogen gas at 0° and 760 m.m. will be produced 



by dissolving 6.5 grams of zinc in dilute sulphuric acid? 



239. What volume of oxygen gas at 15° and 740 m.m. can be obtained 



by heating i gram of red oxide of mercury ? 



240. What volume of hydrogen gas at 100° will unite with 10 c.c. of 



oxygen gas at the same temperature, and what volume of water 

 vapour at 100° will be produced ? 



241. When I c.c. of water at 4° C. is converted into steam at 100° C, 



what volume will the steam occupy under normal atmospheric 

 pressure? 



242. To obtain i liter of chlorine gas, at 15° and 750 m.m., from hydro- 



chloric acid, what weight of black oxide of manganese must be 

 employed ? 



243. What is meant by "valency"? State the valency of chlorine, 



nitrogen, carbon, and oxygen, illustrating your answer by the 

 formuloe of compounds these elements form with hydrogen. 



