Kamis, 05 Juli 2012

Analisis Gravimetri


Hai sobat semua, hari ini kita akan coba membahas tentang konsep analisis gravimetric ^^ mungkin informasinya sangat sederhana sih, karena tidak mungkin saya publish semua, terlalu rame hehehe, tanpa berlama-lama check it out . .
Kita tahu bahwa dalam analisis kuantitatif selalu memfokuskan pada jumlah atau kuantitas dari sebuah sampel, pengukuran sampel dapat dilakukan dengan menghitung berat zat, menghitung volume atau menghitung konsentrasi. Gravimetri merupakan penetapan kuantitas atau jumlah sampel melalui penghitungan berat zat. Sehingga dalam gravimetri produk harus selalu dalam bentuk padatan (solid).
Alat utama dalam gravimetri adalah timbangan dengan tingkat ketelitian yang baik. Umumnya reaksi kimia tidak dalam ukuran besar seperti kilogram, namun dalam satuan yang lebih kecil seperti gram dan mili gram. Timbangan yang dipergunakan memiliki ketelitian yang tinggi atau kepekaan yang tinggi dan disebut dengan neraca analitik atau analytical balance.
Dalam melakukan analisis dengan teknik gravimetric, kemudahan atau kesukaran dari suatu zat untuk membentuk endapan dapat diketahui dengan melihat kelarutannya atau melihat harga dari hasil kali kelarutan yaitu Ksp. Jika harga Ksp suatu zat kecil maka kita dapat mengetahui bahwa zat tersebut sangat mudah membentuk endapan. Ingat definisi kelarutan; kelarutan suatu zat dalam suatu pelarut adalah jumlah zat tersebut sebanyak-banyaknya yang dapat larut dalam pelarut pada suhu tertentu sehingga larutan tepat jenuh.
Berikut ini sobat bisa melihat klasifikasi endapan yang terukur dan pembagian endapannya,, sehingga sobat tidak kualahan saat melakukan perhitungan ^^

Nah kalo menggunakan pereaksi organik juga bisa terbentuk pengendapan, tergantung logam yang mampu membentuk kompleks tertentu dengan pereaksi organik,

 Dalam reaksi pembentukan endapan, dimana endapan merupakan sampel yang akan kita analisis, maka dengan cermat kita dapat memisahkan endapan dari dari zat-zat lain yang juga turut mengendap. Proses ini cukup sulit dilakukan, namun cara yang paling umum adalah mengoksidasi beberapa zat yang mungkin mengganggu sebelum reaksi pengendapan dilakukan.

Pencucian endapan merupakan tahap selanjutnya, proses pencucian umumnya dilakukan dengan menyaring endapan, yang dilanjutkan dengan membilasnya dengan air. Tahap akhir dari proses ini adalah memurnikan endapan, dengan cara menguapkan zat pelarut atau air yang masih ada didalam sampel, pemanasan atau mengeringkan dalam oven lazim dilakukan. Akhirnya penimbangan sampel dapat dilakukan dan hasil penimbangan adalah kuantitas sampel yang dianalisis.
nah sekarang sobat, kita contoh soal ^^ saya males ngetiknya . . saya sudah snapshot aja ya hehehe . . ntar di publish kok sumbernya, so sobat bisa belajar sendiri,

Soal 1

Soal 2
Soal 3
Soal 4
Soal 5
Soal 6

Semoga bermanfaat sobat ^^ . .

nah, soal latihan buat kamu . . kalo ada yang ga ngerti silahkan postingkan komentar kalian semua dibawah ya, insyaAllah saya bahas, mungkin dari materi atau pembahasan soal ^^


1.           A sample of an impure iron ore is believed to be approximately 55% w/w Fe. The amount of Fe in the sample is to be determined gravimetrically by isolating it as Fe2O3. How many grams of sample should be taken to ensure that approximately 1 g of Fe2O3 will be isolated?
2.           The concentration of arsenic in an insecticide can be determined gravimetrically by precipitating it as MgNH4AsO4. The precipitate is ignited and weighed as Mg2As2O7. Determine the %w/w As2O3 in a 1.627-g sample of insecticide that yields 106.5 mg of Mg2As2O7.
3.           After preparing a sample of alum, K2SO4.Al2(SO4)3.24H2O, a student determined its purity gravimetrically. A 1.2931-g sample was dissolved and the aluminum precipitated as Al(OH)3. The precipitate was collected by filtration, washed, and ignited to Al2O3, yielding 0.1357 g. What is the purity of the alum preparation?
4.           To determine the amount of iron in a dietary supplement, a random sample of 15 tablets weighing a total of 20.505 g was ground into a fine powder. A 3.116-g sample of the powdered tablets was dissolved and treated to precipitate the iron as Fe(OH)3. The precipitate was collected, rinsed, and ignited to  a constant weight as Fe2O3, yielding 0.355 g. Report the iron content of the dietary supplement as g FeSO4.7H2O per tablet.
5.           A 1.4639-g sample of limestone was analyzed for Fe, Ca, and Mg. The iron was determined as Fe2O3, yielding 0.0357 g. Calcium was isolated as CaSO4, yielding a precipitate of 1.4058 g, and Mg was isolated as 0.0672 g of Mg2P2O7. Report the amount of Fe, Ca, and Mg in the limestone sample as %w/w Fe2O3, %w/w CaO, and %w/w MgO.
6.           The number of ethoxy groups (CH3CH2O–) in an organic compound can be determined by the following sequence of reactions 
R(OCH2CH3)x + xHI → R(OH)x + xCH3CH2I
                          CH3CH2I + Ag+ + H2O → AgI(s) + CH3CH2OH
A 36.92-mg sample of an organic compound with an approximate molecular weight of 176 was treated in this fashion, yielding 0.1478 g of AgI. How many ethoxy groups are there in each molecule?
7.           A 516.7-mg sample containing a mixture of K2SO4 and (NH4)2SO4 was dissolved in water and treated with BaCl2, precipitating the SO42– as BaSO4. The resulting precipitate was isolated by filtration, rinsed free of impurities, and dried to a constant weight, yielding 863.5 mg of BaSO4. What is the %w/w K2SO4 in the sample?
8.           The amount of iron and manganese in an alloy can be determined by precipitating the metals with 8- hydroxyquinoline, C9H7NO. After weighing the mixed precipitate, the precipitate is dissolved and the amount of 8-hydroxyquinoline determined by another method. In a typical analysis, a 127.3-mg sample of an alloy containing iron, manganese, and other metals was dissolved in acid and treated with appropriate masking agents to prevent an interference from other metals. The iron and manganese were precipitated and isolated as Fe(C9H6NO)3 and Mn(C9H6NO)2, yielding a total mass of 867.8 mg. The amount of 8-hydroxyquinolate in the mixed precipitate was determined to be 5.276 mmol. Calculate the %w/w Fe and %w/w Mn in the alloy.
9.           A 0.8612-g sample of a mixture consisting of NaBr, NaI, and NaNO3 was analyzed by adding AgNO3 to precipitate the Br – and I–, yielding a 1.0186-g mixture of AgBr and AgI. The precipitate was then heated in a stream of Cl2, converting it to 0.7125 g of AgCl. Calculate the %w/w NaNO3 in the sample.
10.       The earliest determinations of elemental atomic weights were accomplished gravimetrically. In determining the atomic weight of manganese, a carefully purified sample of MnBr2 weighing 7.16539 g was dissolved and the Br precipitated as AgBr, yielding 12.53112 g. What is the atomic weight for Mn if the atomic weights for Ag and Br are taken to be 107.868 and 79.904, respectively?
11.       A sample of impure pyrite known to be approximately 90–95% w/w FeS2 is to be analyzed by oxidizing the sulfur to SO42– and precipitating as BaSO4. How many grams of the sample must be taken if a minimum of 1 g of BaSO4 is desired?
12.       A series of samples consisting of any possible combination of KCl, NaCl, and NH4Cl is to be analyzed by adding AgNO3 to precipitate AgCl. What is the minimum volume of 5% w/v AgNO3 necessary to completely precipitate the chloride in any 0.5-g sample?
13.      A 38.63-mg sample of potassium ozonide, KO3, was heated to 70 °C for 1 h, undergoing a weight loss of 7.10 mg. Write a balanced chemical reaction describing this decomposition reaction. A 29.6-mg sample of impure KO3 experiences a 4.86-mg weight loss when treated under similar condition. What is the %w/w KO3 in the sample?
14.       The water content of an 875.4-mg sample of cheese was determined with a moisture analyzer. What is the %w/w H2O in the cheese if the final mass was found to be 545.8 mg?
15.       The iron content of an organometallic compound was determined by treating a 0.4873-g sample with HNO3 and heating to volatilize the organic material. After ignition, the residue of Fe2O3 was found to weigh 0.2091 g. (a) What is the %w/w Fe in this compound? The carbon and hydrogen in a second sample of the compound were determined by a combustion analysis. When a 0.5123-g sample was carried through the analysis, 1.2119 g of CO2 and 0.2482 g of H2O were collected. (b) What are the %w/w C and %w/w H in this compound? (c) What is the compound’s empirical formula?




Source :
http://www.chem-is-try.org/materi_kimia/kimia-kesehatan/pemisahan-kimia-dan-analisis/gravimetri/

David Harvey, 2000, Modern Analytical Chemistry.

1 komentar:

  1. lanjutkan demi kemajuan bangsa,, karena tak ada yang lahir dalam keadaan pandai..

    BalasHapus