chemistry (help) F.4

2009-01-03 10:39 pm

When magnesium is heated in air, it combines with oxygen to form magnesium oxide. A student heated the magnesium ribbon in a crucible until ignitited. The weight of the apparatus and materials was recorded.

weight of crucible = 37.27g
weight of crucible and magnesium ribbon = 40.89g
weight of crucible and product = 45.69g

(a) Find the mole ratio of magnesium to oxygen in the product.
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我計到個mole ratio 是1:3. 因為…
mg oxygen
mass: 3.62 4.8
no. of mole: (3.62/24.3) 0.3
mole ratio: 1 : 3

明明molar mass of magnesium 是24.3，但model answer 話是24, i know ans 是correct 了to nearest integer。但questin 根本沒有話要correct, 所以個ans 是 (3.62/24) --> mole ratio = 1:2 . 如果考試佢又冇寫清楚ge話，咁咪會計錯1:3 囉？！因為1:2 同1:3 的相差都頗大的！

回答 (1)

老爺子

2009-01-04 12:38 am

✔ 最佳答案

第一，個答案叫做佢俾既 answer，唔好叫做 model answer。我讀大學既時候，教授都只會叫自己既答案叫 suggested answer，唔會叫自己出既答案做 model answer 咁自大。

第二，這類計算題目，應同時俾埋相關的 relative atomic masses，唔會要你背埋全部 relative atomic masses 去做題目。如果係會考，會成個 Periodic Table 俾埋你，裡面會有atomic number 同埋 relative atomic masses，而且其他 constants (e.g. Avogadro constant) 都會俾埋，所以唔駛担心。
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