Ta có phương trình:

\(3Fe+2O_2\rightarrow Fe_3O_4\)

\(nFe=\dfrac{13,2}{56}=\dfrac{33}{140}\left(mol\right)\)

\(\Rightarrow nFe_3O_4=\dfrac{33}{140}:3=\dfrac{11}{140}\left(mol\right)\)

\(\Rightarrow mFe_3O_4=\dfrac{11}{140}.232=\dfrac{638}{35}\left(g\right)\)

a, -78(31+24+17)+22.72=-78.72+22.72=72.(-78+22)=-4032

b, -15,5.(20,8+9,2)+3,5.(9,2+20,8)=(-15,5+3,5).(20,8+9,2)=-12.30=-360

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nhôm nha bn

a,\(\dfrac{2^{19}.27^3+15.4^9.9^4}{6^9.2^{10}+12^{10}}\)=\(\dfrac{2^{19}.\left(3^3\right)^3+3.5.\left(2^2\right)^9.\left(3^2\right)^4}{6^9.2^{10}+6^{10}.2^{10}}\)

=\(\dfrac{2^{19}.3^9+2^{18}.3^9.5}{6^9.2^{10}.\left(1+6\right)}\)=\(\dfrac{2^{18}.3^9.\left(2+5\right)}{6^9.2^{10}.7}\)=\(\dfrac{2^{18}.3^9}{6^9.2^{10}}=\dfrac{2^{10}.2^8.3^9}{2^9.3^9.2^{10}}=\dfrac{2^8}{2^8.2}=\dfrac{1}{2}\)

b, \(\dfrac{\left(\dfrac{-1}{2}\right)^3-\left(\dfrac{3}{4}\right)^3.\left(-2\right)^2}{2.\left(-1\right)^5+\left(\dfrac{3}{4}\right)^2-\dfrac{3}{8}}=\dfrac{\dfrac{-1}{8}-\dfrac{27}{64}.4}{-2+\dfrac{9}{16}-\dfrac{3}{8}}=\dfrac{\dfrac{-1}{8}-\dfrac{27}{16}}{\dfrac{-23}{16}-\dfrac{3}{8}}=\dfrac{\dfrac{-29}{16}}{\dfrac{-29}{16}}=1\)

dễ lắm!

a, 1/2

b, 1

(\(3\dfrac{1}{3}:3,2+4,5.1\dfrac{31}{45}\)):(\(-21\dfrac{1}{2}\))

=(\(\dfrac{10}{3}:3,2+4,5.\dfrac{76}{45}\)):(\(-\dfrac{43}{2}\))

=(\(\dfrac{25}{24}+\dfrac{38}{5}\)):(\(-\dfrac{43}{2}\))

=\(\dfrac{-1037}{2580}\)

1, I never go to school by bike.

2, At the moment, I am reading a book and my brother

3, Hoa lives in HN and Ha lives in HCM City.

4, They usually get up at 6.00 in the morning.

5, My teacher tells Hoa about Math.

PT: 3Fe + 2O₂ ------> Fe₃O₄

nFe=\(\dfrac{8,4}{56}=0,15\left(mol\right)\)

nO₂= \(\dfrac{0,15.3}{2}=0,225\left(mol\right)\)=>VO₂=0,225. 22,4=5,04(lit)

nFe₃O₄= 0,15: 3=0,05=>mFe₃O₄=0,05.232=11,6 g