用14.2g无水Na2SO4配制成500mL溶液，其物质的量浓度为多少？若从中取

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问题问答题

用14.2g无水Na₂SO₄配制成500mL溶液，其物质的量浓度为多少？若从中取出10mL，含溶质Na₂SO₄的质量是多少克？

答案

n（Na₂SO₄）=

14.2g

142g/mol

=0.1mol，

c（Na₂SO₄）=

0.1mol

0.5L

=0.2mol/L，

若从中取出10mL，含溶质Na₂SO₄的质量是：m=n?M=0.01L×0.2mol/L×142g/mol=0.284g，

答：物质的量浓度为0.2mol/L，若从中取出10mL，含溶质Na₂SO₄的质量是0.284g．

选择题

They _____ on the program for almost one week before I joined them, and now, we ____on it as no good results have come out so far.

A．had been working; are still working

B．had worked; were still working

C．have been working; have worked

D．have worked; are still working

单项选择题

以下（）不会造成手动变速器跳档。

A.自锁装置损坏

B.啮合齿轮磨损过甚

C.变速器轴承磨损过甚造成过大旷动

D.换挡杆调整不当