17.2 g CaSO4·xH2O转变成2CaSO4·(x-1)H2O时，质量减少2.7 g，则x值是( )
A．1
B．2
C．4
D．6
答案：B
写出反应方程式，根据差量法计算。
2(CaSO4·xH2O)
2CaSO4·(x-1)H2O+(x+1)H2O
2(136+18x) g 18(x+1) g
17.2 g 2.7g
求得 x=2。
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阅读短文，从下列所给的五个句子中，选择四句还原到短文中，使短文意思通顺、结构完整。
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| Mabel Welton,62,was in her living room when a piece of dark gray metal hit the roof of her home. 1 . Because her house in Hassle,UK lies under a flight path. However,after examining the piece,the Royal Air Force( RAF) said that the metal was space junk that fell to Earth. 2 . "We heard this sound and went outside and saw this hole in the roof,"she said. "The metal had broken two tiles(瓦片) and made a mess. " Welton said that her granddaughter's boyfriend Richard Sutcliffe had to"put on some oven gloves to get it because it was so hot". The Weltons gave the metal to the police. 3 . Last Thuraday the RAF said they learned that the metal was space rubbish. 4 . A great number of pieces from more than 5,000 old satellites are now in space. Most pieces have fallen to Earth without doing damage to people or houses. |