環己烷溶液凝固點測定及溶質分子量測定實驗之改進研究

Experimental Conditions and Techniques in the Determination of Freezing Point Depression of Cyclohexane Solutions

佘瑞琳(Jul-Lin She) ； 鄭淑芬(Soo-Fin Cheng) ； 陳竹亭(Jwu-Ting Chen)

《化學》 48卷3期 (1990/09) Pp. 219-225

https://doi.org/10.6623/chem.1990027

摘要

利用含可溶性物質的苯或環己烷溶液之凝固點下降來測定溶質的分子量，是一個常見的普通化學實驗。但是由於苯的毒性高，理想的環己烷溶液冷卻曲線又不易測得；因此在本研究中欲探討使用環己烷取代毒性較高的苯，作爲測定溶液凝固點下降之溶劑的適當實驗條件，期能適合目前普通化學實驗使用。實驗結果顯示：當使用内、外管管徑分別爲25及30mm的測定装置，待測分子量之溶質選用萘、二聯苯及二苯基酮，混合溶液的濃度爲1~2wt%，冷卻系統採用溫度維持在0℃的冰水浴時，可得到良好的溶液冷卻曲線；由此曲線作圖及經計算而得之溶質分子量誤差可在5%以内，測定一組數據所需時間爲10~12分鐘。問巻調査1,422位學生以此實驗條件試做實驗的結果爲：84%的學生可做出理想的純環己烷冷卻曲線，能做出環己烷溶液的理想冷卻曲線者有67%，顯示溶液的凝固點較不易測定；學生經測定及計算所得未知物溶質分子量誤差範圍在10%以内者，佔54%；全班做完此實驗所需時間約爲2.5小時。此外，當測定装置若配合原有之内管，改變外管管徑爲35、40及45mm，同樣使用冰水浴冷卻時，測定之準確性不受内、外管間空氣夾層的不同影響，但測定一組數據所需時間增長至15~20分鐘，較不適合三小時普通化學實驗的時間。然而，外管管徑大者，如35、40及45mm，若改使用溫度約爲-10℃、溫差較大的冰鹽浴冷卻，溶質萘的濃度爲1~3wt %時，測定一組數據所需時間均縮短至10分鐘以内，測定所得溶質分子量之誤差小於5%。硏究結果均顯示本實驗經上述改進後將更適合普通化學實驗。

並列摘要

The purpose of this study is to investigate the experimental conditions and techniques for the determination of the freezing point depression of cyclohexane solutions. and the determination of molecular weights of the unknown solutes. so that such experiment will be suitable for the general chemistry laboratory. It was found that the molecular weight of the solutes within 5 % error. and the proper depression curves could be obtained by employing (1nhe inner- and outer-test tubes of 25 mm and 30 mm. respectively, (2)an ice-water bath of 0 °C for cooling, and (3)cyclohexane solutions containing 1 or 2 wt% of naphthalene, diphenyl, and benzophenone. The time consumed for collecting a set of data of a depression curve would be 10-12 miniutes, And, the whole experiment could be finished within 2.5 hours. The larger outer test tubes with diameters in 35, 40 and 45 mm could be used as well. when ice-salt bath of -10℃ was used as the cooling system. The performance of 1422 students of this experiment was evaluated. The results showed that 54 % of the students could determine the mloecular weights of unknown solutes with an error less than 10 %. Most of the students completed the experiment within 2.5 hours.