搜题请搜索小程序“爱下问搜题”
问题 选择题

若将阿伏加德罗常数规定为0.024 kg 12C中所含有的C原子数，而其他概念和规定一律不变，则下列各项的数值不会发生变化的是(    )

A．物质的量

B．相对原子质量

C．物质的摩尔质量

D．12 g12C中所含有的C原子数

答案

BD

由公式n=N/NA知，NA发生了变化，则N必发生变化，A选项不可选。又由于n=,而不论对NA如何规定，一定量的物质的质量是不变的，则M="mn" 随n的变化而变化。C选项不可选。

相对原子质量有它自己的标准(12C原子质量的1/12)，只随其标准的改变而改变，不随阿伏加德罗常数的改变而改变，B选项可选。只要物质的质量(或体积)一定，则构成它的粒子数一定，不随阿伏加德罗常数的改变而改变，D选项可选。

多项选择题

The Cosmetic Co is a company producing a variety of cosmetic creams and lotions. The creams and lotions are sold to a variety of retailers at a price of $23·20 for each jar of face cream and $16·80 for each bottle of body lotion. Each of the products has a variety of ingredients, with the key ones being silk powder, silk amino acids and aloe vera. Six months ago, silk worms were attacked by disease causing a huge reduction in the availability of silk powder and silk amino acids. The Cosmetic Co had to dramatically reduce production and make part of its workforce, which it had trained over a number of years, redundant.The company now wants to increase production again by ensuring that it uses the limited ingredients available to maximise profits by selling the optimum mix of creams and lotions. Due to the redundancies made earlier in the year, supply of skilled labour is now limited in the short-term to 160 hours (9,600 minutes) per week, although unskilled labour is unlimited. The purchasing manager is confident that they can obtain 5,000 grams of silk powder and 1,600 grams of silk amino acids per week. All other ingredients are unlimited. The following information is available for the two products:Cream LotionMaterials required: silk powder (at $2·20 per gram) 3 grams 2 grams– silk amino acids (at $0·80 per gram) 1 gram 0·5 grams– aloe vera (at $1·40 per gram) 4 grams 2 gramsLabour required: skilled ($12 per hour)4 minutes 5 minutes– unskilled (at $8 per hour) 3 minutes 1·5 minutesEach jar of cream sold generates a contribution of $9 per unit, whilst each bottle of lotion generates a contribution of $8 per unit. The maximum demand for lotions is 2,000 bottles per week, although demand for creams is unlimited. Fixed costs total $1,800 per week. The company does not keep inventory although if a product is partially complete at the end of one week, its production will be completed in the following week.Required:

(b)Calculate the shadow price for silk powder and the slack for silk amino acids. All workings MUST be rounded to 2 decimal places. (6 marks)
查看答案
选择题
如图所示，两根相距为l的平行直导轨ab、cd、b、d间连有一固定电阻R，导轨电阻可忽略不计；MN为放在ab和cd上的一导体杆，与ab垂直，其电阻也为R；整个装置处于匀强磁场中，磁感应强度的大小为B，磁场方向垂直于导轨所在平面（指向图中纸面内）；现对MN施力使它沿导轨方向以速度v（如图）做匀速运动；令U表示MN两端电压的大小，则（　　）

A．U=vBl，流过固定电阻R的感应电流由b到d

B．U=vBl，流过固定电阻R的感应电流由d到b

C．U=vBl，流过固定电阻R的感应电流由b到d

D．U=vBl，流过固定电阻R的感应电流由d到b
查看答案