下列溶液中微粒的物质的量浓度关系正确的是 A．25℃时，pH=7_爱下问搜题

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问题选择题

下列溶液中微粒的物质的量浓度关系正确的是

A．25℃时，pH=7的NH₄Cl与NH₃•H₂O混合溶液：c（H⁺）=c（OH^-）=c（NH₄⁺）=c（Cl^-）

B．0.1mol/LNa₂S溶液：c（OH^-）=c（H⁺）+c（HS^-）+c（H₂S）

C．25℃时，pH=2的CH₃COOH与pH=12的NaOH等体积混合： c（CH₃COO^-）+c（H⁺）＜c（Na⁺）+c（OH^-）

D．0.1mol/LNa₂CO₃溶液：2c（CO₃^2-）+2c（HCO^-）+2c（H₂CO₃）=c（Na⁺）

答案

答案：D

题目分析：A、中性溶液中有c（H⁺）=c（OH^-），又符合电荷守恒，c（H⁺）+ c（NH₄⁺）= c（OH^-）+ c（Cl^-），所以有c（NH₄⁺）=c（Cl^-）> c（H⁺）=c（OH^-）,错误；B、0.1mol/LNa₂S溶液中，根据质子守恒得c（OH^-）=c（H⁺）+c（HS^-）+2c（H₂S），错误；C、25℃时，pH=2的CH₃COOH与pH=12的NaOH溶液中，c(CH₃COOH)>c(NaOH),等体积混合后的溶液实为CH₃COOH和CH₃COONa所以c（CH₃COO^-）+c（H⁺）>c（Na⁺）+c（OH^-）,错误；D、根据元素守恒得2c（CO₃^2- ）+2c（HCO^- ）+2c（H₂CO₃）=（Na⁺），正确，答案选D。

单项选择题

企业投资项目的最终目的是为了追求财务效益、经济效益的最大化，而社会公共项目追求（）的平衡。

A．财务效益与经济效益

B．财务效益与社会效益

C．经济效益与社会效益

D．社会效益与安全效益

单项选择题

Space travel is definitely bad for astronauts’ bones, reducing their bone density after only a month of weightlessness, according to French research published on Friday.
Laurence Vico and his fellow workers at St Etienne University called for more research into the effects of microgravity, after their study of 15 astronauts from the Russian MIR station showed bone loss continued throughout space flights.
"Bone loss was especially striking in four astronauts, " the scientists reported in the Lancet Medical Journal.
They measured the Bone Mineral Density (BMD) of bones in the forearm and lower leg of the astronauts who had spent one to six months in space.
The BMD loss was significant in the tibia of the lower leg, a weight-bearing bone, but barely changed in the radius of the forearm. "Our results indicate the need to investigate not only different bones, but also different areas of the same bone since not all sites of the skeleton are similarly affected by space conditions, " they added.
Without gravity the body isn’t bearing any weight so there is no need for calcium which makes bones p, and it becomes empty into the bloodstream.
The research team suggested in future scientists should try to determine if the loss of bone density was only on weight-bearing bones on longer flights, also the possible recovery after returning to Earth.

Scientists have found that ______.

A. the BMD loss may cause serious illness to astronauts
B. the BMD loss may cause some change in astronauts’ bodies
C. astronauts shouldn’t care about the BMD loss
D. astronauts should take some calcium before space travel