a) \(m_O=\dfrac{20.20}{100}=4\left(g\right)\)

=> \(n_{CaO}=n_O=\dfrac{4}{16}=0,25\left(mol\right)\)

\(\left\{{}\begin{matrix}\%m_{CaO}=\dfrac{0,25.56}{20}.100\%=70\%\\\%m_{Ca}=100\%-70\%=30\%\end{matrix}\right.\)

b) \(n_{Ca}=\dfrac{20.30\%}{40}=0,15\left(mol\right)\)

PTHH: Ca+ 2H₂O --> Ca(OH)₂ + H₂

          0,15-------------------->0,15

=> V = 0,15.22,4 = 3,36 (l)

\(n_{Fe_3O_4}=\dfrac{23,2}{232}=0,1\left(mol\right)\)

=> n_Fe = 0,3 (mol)

=> m_Fe = 0,3.56 = 16,8 (g)

=> \(m=\dfrac{16,8.100}{78,9474}=21,28\left(g\right)\)

c) Giả sử Fe₃O₄ bị khử thành Fe

Gọi số mol Fe₃O₄ pư là a (mol)

PTHH: Fe₃O₄ + 4H₂ --> 3Fe + 4H₂O

                 a--->4a----->3a

Xét tỉ lệ: \(\dfrac{0,1}{1}>\dfrac{0,15}{4}\) => Hiệu suất tính theo H₂

m = 23,2 - 232a + 168a = 21,28

=> a = 0,03 (mol)

=> \(\left\{{}\begin{matrix}n_{Fe_3O_4\left(pư\right)}=0,03\left(mol\right)\\n_{H_2\left(pư\right)}=0,12\left(mol\right)\end{matrix}\right.\)

\(H=\dfrac{n_{H_2\left(pư\right)}}{n_{H_2\left(bđ\right)}}=\dfrac{0,12}{0,15}.100\%=80\%\)

8 How much do these apples cost?

9 THis is a blue car

11 Are there 40 classrooms in Phong's school?

13 How wide if the Great wall

cảm ơn bạn ạ

a computer is used to do that job nowadays

He will be seen off at the airport by all his friends 

Beer used to be drunk for breakfast in England years ago

Tea can not made with cold water.

the floor was being cleaned when i arrived

Should Julia be helped with the sewing ?

4 There aren't any tomatoes left

5 My sister's favorite food is chicken

Chưa cả kịp đọc cái bài mà dưới đã...

không biết xét dấu của phần c đúng chưa,tại m mới học 

với cả m làm hơi ẩu ,có gì thông cảm

phần d thì làm tương tự phần c nhé!!!

8. Her telephone number isn't known by me.

9. The children will be brought home by my students.

10. I was sent a present last week.

11. More information was given to us by her.

12. All the workers of the plan were being instructed by the chief engineer.

 I hadn’t been told about it.

. Her telephone number isn’t known.

The children will be brought home by my students. 

I was sent a present last week.

. More information was given us. 

All the workers were being instructed of the plan by the chief engineer.

\(5;;\sqrt{\left(x+5\right)\left(3x+4\right)}>4\left(x-1\right)\)

\(\Leftrightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}4\left(x-1\right)\le0\\\left(x+5\right)\left(3x+4\right)\ge0\end{matrix}\right.\\\left\{{}\begin{matrix}4\left(x-1\right)\ge0\\\left(x+5\right)\left(3x+4\right)\ge0\\\left(x+5\right)\left(3x+4\right)>16\left(x-1\right)^2\end{matrix}\right.\end{matrix}\right.\)

\(TH:\left\{{}\begin{matrix}4\left(x-1\right)\le0\\\left(x+5\right)\left(3x+4\right)\ge0\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x\le1\\\left[{}\begin{matrix}x\le-5\\x\ge-\dfrac{4}{3}\end{matrix}\right.\end{matrix}\right.\)

\(\Leftrightarrow x\in(-\infty;-5]\cup\left[-\dfrac{4}{3};1\right]\left(1\right)\)

\(TH:\left\{{}\begin{matrix}4\left(x-1\right)\ge0\\\left(x+5\right)\left(3x+4\right)\ge0\\\left(x+5\right)\left(3x+4\right)>16\left(x-1\right)^2\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x\ge1\\\left[{}\begin{matrix}x\le-5\\x\ge-\dfrac{4}{3}\end{matrix}\right.\\-\dfrac{1}{13}< x< 4\\\end{matrix}\right.\)\(\Rightarrow x\in[1;4)\left(2\right)\)

\(\left(1\right)\left(2\right)\Rightarrow x\in(-\infty;5]\cup[\dfrac{-4}{3};4)\)

\(6;;;;\sqrt{7x+7}+\sqrt{7x-6}+2\sqrt{49x^2+7x-42}< 181-14x\)

(đoạn 49x^2+7x+42 chắc bạn viết sai đề dấu"-" thành "+")

\(đk:\left\{{}\begin{matrix}7x+7\ge0\\7x-6\ge0\end{matrix}\right.\) \(\Leftrightarrow x\ge\dfrac{6}{7}\)

\(bpt\Leftrightarrow\sqrt{7x+7}+\sqrt{7x-6}+2\sqrt{\left(7x+7\right)\left(7x-6\right)}+14x+1< 182\left(1\right)\)

\(đặt:\sqrt{7x+7}+\sqrt{7x-6}=t>0\)

\(\Rightarrow t^2=14x+1+2\sqrt{\left(7x+7\right)\left(7x-6\right)}\)

\(\Rightarrow\left(1\right)\Leftrightarrow t^2+t< 182\Leftrightarrow-14< t< 13\)

\(\Rightarrow\sqrt{7x+7}+\sqrt{7x-6}< 13\Leftrightarrow14x+1+2\sqrt{\left(7x+7\right)\left(7x-6\right)}< 169\)

\(\Leftrightarrow2\sqrt{\left(7x+7\right)\left(7x-6\right)}< 168-14x\)

\(\Leftrightarrow\left\{{}\begin{matrix}168-14x\ge0\\\left(7x+7\right)\left(7x-6\right)\ge0\\4\left(7x+7\right)\left(7x-6\right)< \left(168-14x\right)^2\\\end{matrix}\right.\)

\(\Leftrightarrow\left\{{}\begin{matrix}x\le12\\\left[{}\begin{matrix}x\le-1\\x\ge\dfrac{6}{7}\end{matrix}\right.\\x< 6\\\end{matrix}\right.\)\(\Rightarrow\dfrac{6}{7}\le x< 6\)

22. The victims have been provided with food and clothing.
23. English is spoken in almost every corner of the world.
24. This machine mustn't be used after 5:30 pm.
25. After class, the chalk board is always erased by one of the students.